binding: mdio-mux: Add DT binding doc for Broadcom MDIO bus multiplexer

Properties for an MDIO bus multiplexer found in Broadcom iProc based SoCs.

This MDIO bus multiplexer defines buses that could be internal as well as

external to SoCs and could accept MDIO transaction compatible to C-22 or

C-45 Clause. When child bus is selected, one needs to select these two

properties as well to generate desired MDIO transaction on appropriate bus.

Required properties in addition to the generic multiplexer properties:

MDIO multiplexer node:

- compatible: brcm,mdio-mux-iproc.

Every non-ethernet PHY requires a compatible so that it could be probed based

on this compatible string.

Additional information regarding generic multiplexer properties can be found

at- Documentation/devicetree/bindings/net/mdio-mux.txt

for example:

		mdio_mux_iproc: mdio-mux@6602023c {

			compatible = "brcm,mdio-mux-iproc";

			reg = <0x6602023c 0x14>;

			#address-cells = <1>;

			#size-cells = <0>;

			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>;

				};

			};

			mdio@7 {

				reg = <0x7>;

				#address-cells = <1>;

				#size-cells = <0>;

				pci_phy1: pci-phy@0 {

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

					reg = <0x0>;

					#phy-cells = <0>;

				};

			};

			mdio@10 {

				reg = <0x10>;

				#address-cells = <1>;

				#size-cells = <0>;

				gphy0: eth-phy@10 {

					reg = <0x10>;

				};

			};

		};