APM X-Gene Standby GPIO controller bindings

This is a gpio controller in the standby domain. It also supports interrupt in
some particular pins which are sourced to its parent interrupt controller
as diagram below:
                            +-----------------+
                            | X-Gene standby  |
                            | GPIO controller +------ GPIO_0
+------------+              |                 | ...
| Parent IRQ | EXT_INT_0    |                 +------ GPIO_8/EXT_INT_0
| controller | (SPI40)      |                 | ...
| (GICv2)    +--------------+                 +------ GPIO_[N+8]/EXT_INT_N
|            |   ...        |                 |
|            | EXT_INT_N    |                 +------ GPIO_[N+9]
|            | (SPI[40 + N])|                 | ...
|            +--------------+                 +------ GPIO_MAX
+------------+              +-----------------+

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: The EXT_INT_0 parent interrupt resource must be listed first.
- interrupt-cells: Should be two.
       - first cell is 0-N corresponding for EXT_INT_0 to EXT_INT_N.
       - second cell is used to specify flags.
- interrupt-controller: Marks the device node as an interrupt controller.
- apm,nr-gpios: Optional, specify number of gpios pin.
- apm,nr-irqs: Optional, specify number of interrupt pins.
- apm,irq-start: Optional, specify lowest gpio pin support interrupt.

Example:
	sbgpio: gpio@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>;
		interrupt-parent = <&gic>;
		#interrupt-cells = <2>;
		interrupt-controller;
		apm,nr-gpios = <22>;
		apm,nr-irqs = <6>;
		apm,irq-start = <8>;
	};

	testuser {
		compatible = "example,testuser";
		/* Use the GPIO_13/EXT_INT_5 line as an active high triggered
		 * level interrupt
		 */
		interrupts = <5 4>;
		interrupt-parent = <&sbgpio>;
	};
