net: dsa: Add Microchip KSZ switches binding

A sample SPI configuration for Microchip KSZ switches.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/ksz.txt

+Microchip KSZ Series Ethernet switches
+==================================
+
+Required properties:
+
+- compatible: For external switch chips, compatible string must be exactly one
+  of: "microchip,ksz9477"
+
+See Documentation/devicetree/bindings/dsa/dsa.txt for a list of additional
+required and optional properties.
+
+Examples:
+
+Ethernet switch connected via SPI to the host, CPU port wired to eth0:
+
+                             eth0: ethernet@10001000 {
+                                             fixed-link {
+                                                             speed = <1000>;
+                                                             full-duplex;
+                                             };
+                             };
+
+                             spi1: spi@f8008000 {
+                                             pinctrl-0 = <&pinctrl_spi_ksz>;
+                                             cs-gpios = <&pioC 25 0>;
+                                             id = <1>;
+                                             status = "okay";
+
+                                             ksz9477: ksz9477@0 {
+                                                             compatible = "microchip,ksz9477";
+                                                             reg = <0>;
+
+                                                             spi-max-frequency = <44000000>;
+                                                             spi-cpha;
+                                                             spi-cpol;
+
+                                                             status = "okay";
+                                                             ports {
+                                                                             #address-cells = <1>;
+                                                                             #size-cells = <0>;
+                                                                             port@0 {
+                                                                                             reg = <0>;
+                                                                                             label = "lan1";
+                                                                             };
+                                                                             port@1 {
+                                                                                             reg = <1>;
+                                                                                             label = "lan2";
+                                                                             };
+                                                                             port@2 {
+                                                                                             reg = <2>;
+                                                                                             label = "lan3";
+                                                                             };
+                                                                             port@3 {
+                                                                                             reg = <3>;
+                                                                                             label = "lan4";
+                                                                             };
+                                                                             port@4 {
+                                                                                             reg = <4>;
+                                                                                             label = "lan5";
+                                                                             };
+                                                                             port@5 {
+                                                                                             reg = <5>;
+                                                                                             label = "cpu";
+                                                                                             ethernet = <&eth0>;
+                                                                                             fixed-link {
+                                                                                                             speed = <1000>;
+                                                                                                             full-duplex;
+                                                                                             };
+                                                                             };
+                                                             };
+                                             };
+                             };