Documentation: add Device tree bindings for QCA7000

This patch adds the Device tree bindings for the
Ethernet over SPI protocol driver of the Qualcomm
QCA7000 HomePlug GreenPHY.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/qca-qca7000-spi.txt

+* Qualcomm QCA7000 (Ethernet over SPI protocol)
+
+Note: The QCA7000 is useable as a SPI device. In this case it must be defined
+as a child of a SPI master in the device tree.
+
+Required properties:
+- compatible : Should be "qca,qca7000"
+- reg : Should specify the SPI chip select
+- interrupts : The first cell should specify the index of the source interrupt
+  and the second cell should specify the trigger type as rising edge
+- spi-cpha : Must be set
+- spi-cpol: Must be set
+
+Optional properties:
+- interrupt-parent : Specify the pHandle of the source interrupt
+- spi-max-frequency : Maximum frequency of the SPI bus the chip can operate at.
+  Numbers smaller than 1000000 or greater than 16000000 are invalid. Missing
+  the property will set the SPI frequency to 8000000 Hertz.
+- local-mac-address: 6 bytes, MAC address
+- qca,legacy-mode : Set the SPI data transfer of the QCA7000 to legacy mode.
+  In this mode the SPI master must toggle the chip select between each data
+  word. In burst mode these gaps aren't necessary, which is faster.
+  This setting depends on how the QCA7000 is setup via GPIO pin strapping.
+  If the property is missing the driver defaults to burst mode.
+
+Example:
+
+/* Freescale i.MX28 SPI master*/
+ssp2: spi@80014000 {
+	#address-cells = <1>;
+	#size-cells = <0>;
+	compatible = "fsl,imx28-spi";
+	pinctrl-names = "default";
+	pinctrl-0 = <&spi2_pins_a>;
+	status = "okay";
+
+	qca7000: ethernet@0 {
+		compatible = "qca,qca7000";
+		reg = <0x0>;
+		interrupt-parent = <&gpio3>;      /* GPIO Bank 3 */
+		interrupts = <25 0x1>;            /* Index: 25, rising edge */
+		spi-cpha;                         /* SPI mode: CPHA=1 */
+		spi-cpol;                         /* SPI mode: CPOL=1 */
+		spi-max-frequency = <8000000>;    /* freq: 8 MHz */
+		local-mac-address = [ A0 B0 C0 D0 E0 F0 ];
+	};
+};