Documentation: add Device tree bindings for Hisilicon hix5hd2 ethernet

Hisilicon hix5hd2 gmac controller

Required properties:

- compatible: should be "hisilicon,hix5hd2-gmac".

- reg: specifies base physical address(s) and size of the device registers.

  The first region is the MAC register base and size.

  The second region is external interface control register.

- interrupts: should contain the MAC interrupt.

- #address-cells: must be <1>.

- #size-cells: must be <0>.

- phy-mode: see ethernet.txt [1].

- phy-handle: see ethernet.txt [1].

- mac-address: see ethernet.txt [1].

- clocks: clock phandle and specifier pair.

- PHY subnode: inherits from phy binding [2]

[1] Documentation/devicetree/bindings/net/ethernet.txt

[2] Documentation/devicetree/bindings/net/phy.txt

Example:

	gmac0: ethernet@f9840000 {

		compatible = "hisilicon,hix5hd2-gmac";

		reg = <0xf9840000 0x1000>,<0xf984300c 0x4>;

		interrupts = <0 71 4>;

		#address-cells = <1>;

		#size-cells = <0>;

		phy-mode = "mii";

		phy-handle = <&phy2>;

		mac-address = [00 00 00 00 00 00];

		clocks = <&clock HIX5HD2_MAC0_CLK>;

		phy2: ethernet-phy@2 {

			reg = <2>;

		};

	};