dt-bindings: net: add qca,ar71xx.txt documentation

Add binding documentation for Atheros/QCA networking IP core used
in many routers.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/qca,ar71xx.txt

+Required properties:
+- compatible:	Should be "qca,<soc>-eth". Currently support compatibles are:
+		qca,ar7100-eth - Atheros AR7100
+		qca,ar7240-eth - Atheros AR7240
+		qca,ar7241-eth - Atheros AR7241
+		qca,ar7242-eth - Atheros AR7242
+		qca,ar9130-eth - Atheros AR9130
+		qca,ar9330-eth - Atheros AR9330
+		qca,ar9340-eth - Atheros AR9340
+		qca,qca9530-eth - Qualcomm Atheros QCA9530
+		qca,qca9550-eth - Qualcomm Atheros QCA9550
+		qca,qca9560-eth - Qualcomm Atheros QCA9560
+
+- reg : Address and length of the register set for the device
+- interrupts : Should contain eth interrupt
+- phy-mode : See ethernet.txt file in the same directory
+- clocks: the clock used by the core
+- clock-names: the names of the clock listed in the clocks property. These are
+	"eth" and "mdio".
+- resets: Should contain phandles to the reset signals
+- reset-names: Should contain the names of reset signal listed in the resets
+		property. These are "mac" and "mdio"
+
+Optional properties:
+- phy-handle : phandle to the PHY device connected to this device.
+- fixed-link : Assume a fixed link. See fixed-link.txt in the same directory.
+  Use instead of phy-handle.
+
+Optional subnodes:
+- mdio : specifies the mdio bus, used as a container for phy nodes
+  according to phy.txt in the same directory
+
+Example:
+
+ethernet@1a000000 {
+	compatible = "qca,ar9330-eth";
+	reg = <0x1a000000 0x200>;
+	interrupts = <5>;
+	resets = <&rst 13>, <&rst 23>;
+	reset-names = "mac", "mdio";
+	clocks = <&pll ATH79_CLK_AHB>, <&pll ATH79_CLK_MDIO>;
+	clock-names = "eth", "mdio";
+
+	phy-mode = "gmii";
+};