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

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";

};