AXP209 GPIO controller

This driver follows the usual GPIO bindings found in
Documentation/devicetree/bindings/gpio/gpio.txt

Required properties:
- compatible: Should be "x-powers,axp209-gpio"
- #gpio-cells: Should be two. The first cell is the pin number and the
  second is the GPIO flags.
- gpio-controller: Marks the device node as a GPIO controller.

This node must be a subnode of the axp20x PMIC, documented in
Documentation/devicetree/bindings/mfd/axp20x.txt

Example:

axp209: pmic@34 {
	compatible = "x-powers,axp209";
	reg = <0x34>;
	interrupt-parent = <&nmi_intc>;
	interrupts = <0 IRQ_TYPE_LEVEL_LOW>;
	interrupt-controller;
	#interrupt-cells = <1>;

	axp_gpio: gpio {
		compatible = "x-powers,axp209-gpio";
		gpio-controller;
		#gpio-cells = <2>;
	};
};