Document: devicetree: add OF documents for lantiq serial port

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: devicetree-discuss@lists.ozlabs.org

Documentation/devicetree/bindings/serial/lantiq_asc.txt

+Lantiq SoC ASC serial controller
+
+Required properties:
+- compatible : Should be "lantiq,asc"
+- reg : Address and length of the register set for the device
+- interrupts: the 3 (tx rx err) interrupt numbers. The interrupt specifier
+  depends on the interrupt-parent interrupt controller.
+
+Example:
+
+asc1: serial@E100C00 {
+	compatible = "lantiq,asc";
+	reg = <0xE100C00 0x400>;
+	interrupt-parent = <&icu0>;
+	interrupts = <112 113 114>;
+};