APM X-Gene SoC MDIO node

MDIO node is defined to describe on-chip MDIO controller.

Required properties:
	- compatible: Must be "apm,xgene-mdio-rgmii" or "apm,xgene-mdio-xfi"
	- #address-cells: Must be <1>.
	- #size-cells: Must be <0>.
	- reg: Address and length of the register set
	- clocks: Reference to the clock entry

For the phys on the mdio bus, there must be a node with the following fields:
	- compatible: PHY identifier.  Please refer ./phy.txt for the format.
	- reg: The ID number for the phy.

Example:

	mdio: mdio@17020000 {
		compatible = "apm,xgene-mdio-rgmii";
		#address-cells = <1>;
		#size-cells = <0>;
		reg = <0x0 0x17020000 0x0 0xd100>;
		clocks = <&menetclk 0>;
	};

	/* Board-specific peripheral configurations */
	&mdio {
		menetphy: phy@3 {
			reg = <0x3>;
		};
		sgenet0phy: phy@4 {
			reg = <0x4>;
		};
		sgenet1phy: phy@5 {
			reg = <0x5>;
		};
	};
