OMAP GPIO controller bindings

Required properties:
- compatible:
  - "ti,omap2-gpio" for OMAP2 controllers
  - "ti,omap3-gpio" for OMAP3 controllers
  - "ti,omap4-gpio" for OMAP4 controllers
- gpio-controller : Marks the device node as a GPIO controller.
- #gpio-cells : Should be two.
  - first cell is the pin number
  - second cell is used to specify optional parameters (unused)
- interrupt-controller: Mark the device node as an interrupt controller.
- #interrupt-cells : Should be 2.
  The first cell is the GPIO number.
  The second cell is used to specify flags:
    bits[3:0] trigger type and level flags:
      1 = low-to-high edge triggered.
      2 = high-to-low edge triggered.
      4 = active high level-sensitive.
      8 = active low level-sensitive.

OMAP specific properties:
- ti,hwmods:		Name of the hwmod associated to the GPIO:
			"gpio<X>", <X> being the 1-based instance number
			from the HW spec.
- ti,gpio-always-on: 	Indicates if a GPIO bank is always powered and
			so will never lose its logic state.


Example:

gpio4: gpio4 {
    compatible = "ti,omap4-gpio";
    ti,hwmods = "gpio4";
    gpio-controller;
    #gpio-cells = <2>;
    interrupt-controller;
    #interrupt-cells = <2>;
};