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

APM X-Gene Standby GPIO controller bindings

This is a gpio controller in the standby domain.

There are 20 GPIO pins from 0..21. There is no GPIO_DS14 or GPIO_DS15,

only GPIO_DS8..GPIO_DS13 support interrupts. The IRQ mapping

is currently 1-to-1 on interrupts 0x28 thru 0x2d.

Required properties:

- compatible: "apm,xgene-gpio-sb" for the X-Gene Standby 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.

- interrupts: Shall contain exactly 6 interrupts.

Example:

	sbgpio: sbgpio@17001000 {

		compatible = "apm,xgene-gpio-sb";

		reg = <0x0 0x17001000 0x0 0x400>;

		#gpio-cells = <2>;

		gpio-controller;

		interrupts = 	<0x0 0x28 0x1>,

				<0x0 0x29 0x1>,

				<0x0 0x2a 0x1>,

				<0x0 0x2b 0x1>,

				<0x0 0x2c 0x1>,

				<0x0 0x2d 0x1>;

	};