dt-bindings: net: Add lantiq, xrx200-net DT bindings

This adds the binding for the PMAC core between the CPU and the GSWIP
switch found on the xrx200 / VR9 Lantiq / Intel SoC.
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: devicetree@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/lantiq,xrx200-net.txt

+Lantiq xRX200 GSWIP PMAC Ethernet driver
+==================================
+
+Required properties:
+
+- compatible	: "lantiq,xrx200-net" for the PMAC of the embedded
+		: GSWIP in the xXR200
+- reg		: memory range of the PMAC core inside of the GSWIP core
+- interrupts	: TX and RX DMA interrupts. Use interrupt-names "tx" for
+		: the TX interrupt and "rx" for the RX interrupt.
+
+Example:
+
+eth0: eth@E10B308 {
+	#address-cells = <1>;
+	#size-cells = <0>;
+	compatible = "lantiq,xrx200-net";
+	reg = <0xE10B308 0x30>;
+	interrupts = <73>, <72>;
+	interrupt-names = "tx", "rx";
+};