Documentation: dtb: xgene: Add MDIO node

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/apm-xgene-mdio.txt

+APM X-Gene SoC MDIO node
+
+MDIO node is defined to describe on-chip MDIO controller.
+
+Required properties:
+	- compatible: Must be "apm,xgene-mdio-rgmii" or "apm,xgene-mdio-xfi"
+	- #address-cells: Must be <1>.
+	- #size-cells: Must be <0>.
+	- reg: Address and length of the register set
+	- clocks: Reference to the clock entry
+
+For the phys on the mdio bus, there must be a node with the following fields:
+	- compatible: PHY identifier.  Please refer ./phy.txt for the format.
+	- reg: The ID number for the phy.
+
+Example:
+
+	mdio: mdio@17020000 {
+		compatible = "apm,xgene-mdio-rgmii";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		reg = <0x0 0x17020000 0x0 0xd100>;
+		clocks = <&menetclk 0>;
+	};
+
+	/* Board-specific peripheral configurations */
+	&mdio {
+		menetphy: phy@3 {
+			reg = <0x3>;
+		};
+		sgenet0phy: phy@4 {
+			reg = <0x4>;
+		};
+		sgenet1phy: phy@5 {
+			reg = <0x5>;
+		};
+	};