Documentation: gpio: Add APM X-Gene SoC GPIO controller DTS binding

APM X-Gene SoC GPIO controller bindings

This is a gpio controller that is part of the flash controller.

This gpio controller controls a total of 48 gpios.

Required properties:

- compatible: "apm,xgene-gpio" for X-Gene GPIO controller

- reg: Physical base address and size of the controller's registers

- #gpio-cells: Should be two.

	- first cell is the pin number

	- second cell is used to specify the gpio polarity:

		0 = active high

		1 = active low

- gpio-controller: Marks the device node as a GPIO controller.

Example:

	gpio0: gpio0@1701c000 {

		compatible = "apm,xgene-gpio";

		reg = <0x0 0x1701c000 0x0 0x40>;

		gpio-controller;

		#gpio-cells = <2>;

	};