Documentation: dts: Add bindings for APM X-Gene SoC ethernet driver

This patch adds documentation for APM X-Gene SoC ethernet DTS binding.
Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Ravi Patel <rapatel@apm.com>
Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

+APM X-Gene SoC Ethernet nodes
+
+Ethernet nodes are defined to describe on-chip ethernet interfaces in
+APM X-Gene SoC.
+
+Required properties:
+- compatible: Should be "apm,xgene-enet"
+- reg: Address and length of the register set for the device. It contains the
+  information of registers in the same order as described by reg-names
+- reg-names: Should contain the register set names
+  - "enet_csr": Ethernet control and status register address space
+  - "ring_csr": Descriptor ring control and status register address space
+  - "ring_cmd": Descriptor ring command register address space
+- interrupts: Ethernet main interrupt
+- clocks: Reference to the clock entry.
+- local-mac-address: MAC address assigned to this device
+- phy-connection-type: Interface type between ethernet device and PHY device
+- phy-handle: Reference to a PHY node connected to this device
+
+- mdio: Device tree subnode with the following required properties:
+  - compatible: Must be "apm,xgene-mdio".
+  - #address-cells: Must be <1>.
+  - #size-cells: Must be <0>.
+
+  For the phy 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.
+
+Optional properties:
+- status: Should be "ok" or "disabled" for enabled/disabled. Default is "ok".
+
+Example:
+	menetclk: menetclk {
+		compatible = "apm,xgene-device-clock";
+		clock-output-names = "menetclk";
+		status = "ok";
+	};
+
+	menet: ethernet@17020000 {
+		compatible = "apm,xgene-enet";
+		status = "disabled";
+		reg = <0x0 0x17020000 0x0 0xd100>,
+		      <0x0 0X17030000 0x0 0X400>,
+		      <0x0 0X10000000 0x0 0X200>;
+		reg-names = "enet_csr", "ring_csr", "ring_cmd";
+		interrupts = <0x0 0x3c 0x4>;
+		clocks = <&menetclk 0>;
+		local-mac-address = [00 01 73 00 00 01];
+		phy-connection-type = "rgmii";
+		phy-handle = <&menetphy>;
+		mdio {
+			compatible = "apm,xgene-mdio";
+			#address-cells = <1>;
+			#size-cells = <0>;
+			menetphy: menetphy@3 {
+				compatible = "ethernet-phy-id001c.c915";
+				reg = <0x3>;
+			};
+
+		};
+	};
+
+/* Board-specific peripheral configurations */
+&menet {
+        status = "ok";
+};