gpio: add bindings for TS-4800 gpio controller

Device tree binding documentation for the TS-4800
GPIO controller.

Signed-off-by: Julien Grossholtz <[e-mail]>
[Point ngpios to gpio.txt]
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/gpio/gpio-ts4800.txt

···
+* 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>;
+};