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.

Optional properties:
- clocks: Should contain frequency clock and gate clock
- clock-names: Should be "freq" and "asc"

Example:

asc0: serial@16600000 {
	compatible = "lantiq,asc";
	reg = <0x16600000 0x100000>;
	interrupt-parent = <&gic>;
	interrupts = <GIC_SHARED 103 IRQ_TYPE_LEVEL_HIGH>,
		<GIC_SHARED 105 IRQ_TYPE_LEVEL_HIGH>,
		<GIC_SHARED 106 IRQ_TYPE_LEVEL_HIGH>;
	clocks = <&cgu CLK_SSX4>, <&cgu GCLK_UART>;
	clock-names = "freq", "asc";
};

asc1: serial@e100c00 {
	compatible = "lantiq,asc";
	reg = <0xE100C00 0x400>;
	interrupt-parent = <&icu0>;
	interrupts = <112 113 114>;
};