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>;
	};
