Document: devicetree: add OF documents for lantiq serial port

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>;

};