dt-bindings: net: Broadcom iProc MDIO bus driver device tree binding

Add device tree binding documentation for the Broadcom iProc MDIO
bus driver.
Signed-off-by: NArun Parameswaran <arunp@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

+* Broadcom iProc MDIO bus controller
+
+Required properties:
+- compatible: should be "brcm,iproc-mdio"
+- reg: address and length of the register set for the MDIO interface
+- #size-cells: must be 1
+- #address-cells: must be 0
+
+Child nodes of this MDIO bus controller node are standard Ethernet PHY device
+nodes as described in Documentation/devicetree/bindings/net/phy.txt
+
+Example:
+
+mdio@18002000 {
+	compatible = "brcm,iproc-mdio";
+	reg = <0x18002000 0x8>;
+	#size-cells = <1>;
+	#address-cells = <0>;
+
+	enet-gphy@0 {
+		reg = <0>;
+	};
+};