binding: PHY: Binding doc for NS2 PCIe PHYs.

* Broadcom NS2 PCIe PHY binding document

Required bus properties:

- reg: MDIO Bus number for the MDIO interface

- #address-cells: must be 1

- #size-cells: must be 0

Required PHY properties:

- compatible: should be "brcm,ns2-pcie-phy"

- reg: MDIO Phy ID for the MDIO interface

- #phy-cells: must be 0

This is a child bus node of "brcm,mdio-mux-iproc" node.

Example:

mdio@0 {

	reg = <0x0>;

	#address-cells = <1>;

	#size-cells = <0>;

	pci_phy0: pci-phy@0 {

		compatible = "brcm,ns2-pcie-phy";

		reg = <0x0>;

		#phy-cells = <0>;

	};

};