gpio: add bindings for TS-4800 gpio controller

* TS-4800 FPGA's GPIO controller bindings

Required properties:

- compatible: Must be "technologic,ts4800-gpio".

- #gpio-cells: Should be two. The first cell is the pin number.

- reg: Physical base address of the controller and length

       of memory mapped region.

Optional property:

- ngpios: See "gpio.txt"

Example:

gpio1: gpio {

	compatible = "technologic,ts4800-gpio";

	reg = <0x10020 0x6>;

	ngpios = <8>;

	gpio-controller;

	#gpio-cells = <2>;

};