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

Add DT binding doc for Broadcom MDIO bus multiplexer driver.
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NPramod Kumar <pramod.kumar@broadcom.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/brcm,mdio-mux-iproc.txt

+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>;
+				};
+			};
+		};