MDIO (Management Data Input/Output) busses

MDIO busses can be described with a node for the MDIO master device
and a set of child nodes for each phy on the bus.

The MDIO node requires the following properties:
- #address-cells  - number of cells required to define phy address on
                    the MDIO bus.
- #size-cells     - should be zero.
- compatible      - name of MDIO bus controller following generic names
                    recommended practice.
- reg	          - address and length of the MDIO register.

Optional property:
- mdio-name       - MDIO bus name

The child nodes of the MDIO driver are the individual PHY devices
connected to this MDIO bus. They must have a "reg" property given the
PHY address on the MDIO bus.
- reg             - (required) phy address in MDIO bus.

Example for cp110 MDIO node at the SoC level:
	cp0_mdio: mdio@12a200 {
		#address-cells = <1>;
		#size-cells = <0>;
		compatible = "marvell,orion-mdio";
		reg = <0x12a200 0x10>;
		mdio-name = "cp0-mdio";
	};

	cp0_xmdio: mdio@12a600 {
		#address-cells = <1>;
		#size-cells = <0>;
		compatible = "marvell,xmdio";
		reg = <0x12a600 0x200>;
		mdio-name = "cp0-xmdio";
	};

And at the board level, example for armada-8040-mcbin board:
	&cp0_mdio {
		ge_phy: ethernet-phy@0 {
			reg = <0>;
		};
	};

	&cp0_xmdio {
		phy0: ethernet-phy@0 {
			reg = <0>;
		};

		phy8: ethernet-phy@8 {
			reg = <8>;
		};
	};
