MDIO on GPIOs

Currently defined compatibles:
- virtual,gpio-mdio

MDC and MDIO lines connected to GPIO controllers are listed in the
gpios property as described in section VIII.1 in the following order:

MDC, MDIO.

Note: Each gpio-mdio bus should have an alias correctly numbered in "aliases"
node.

Example:

aliases {
	mdio-gpio0 = <&mdio0>;
};

mdio0: mdio {
	compatible = "virtual,mdio-gpio";
	#address-cells = <1>;
	#size-cells = <0>;
	gpios = <&qe_pio_a 11
		 &qe_pio_c 6>;
};