Hisilicon Gigabit Ethernet MDIO Controller interface

Required properties:
- compatible: should be "hisilicon,hisi-gemac-mdio".
- reg: address and length of the register set for the device.
- clocks: A phandle to the reference clock for this device.

- PHY subnode: inherits from phy binding [1]
[1] Documentation/devicetree/bindings/net/phy.txt

Example:
mdio: mdio@100503c0 {
	compatible = "hisilicon,hisi-gemac-mdio";
	reg = <0x100503c0 0x20>;
	clocks = <&crg HI3519V100_MDIO_CLK>;
	#address-cells = <1>;
	#size-cells = <0>;

	phy0: phy@1 {
		reg = <1>;
	};
};
