ETRAX FS UART

Required properties:
- compatible : "axis,etraxfs-uart"
- reg: offset and length of the register set for the device.
- interrupts: device interrupt

Optional properties:
- {dtr,dsr,rng,dcd}-gpios: specify a GPIO for DTR/DSR/RI/DCD
  line respectively.

Example:

serial@b00260000 {
	compatible = "axis,etraxfs-uart";
	reg = <0xb0026000 0x1000>;
	interrupts = <68>;
	dtr-gpios = <&sysgpio 0 GPIO_ACTIVE_LOW>;
	dsr-gpios = <&sysgpio 1 GPIO_ACTIVE_LOW>;
	rng-gpios = <&sysgpio 2 GPIO_ACTIVE_LOW>;
	dcd-gpios = <&sysgpio 3 GPIO_ACTIVE_LOW>;
};