NVIDIA Tegra GPIO controller

Required properties:
- compatible : "nvidia,tegra<chip>-gpio"
- reg : Physical base address and length of the controller's registers.
- interrupts : The interrupt outputs from the controller. For Tegra20,
  there should be 7 interrupts specified, and for Tegra30, there should
  be 8 interrupts specified.
- #gpio-cells : Should be two. The first cell is the pin number and the
  second cell is used to specify optional parameters:
  - bit 0 specifies polarity (0 for normal, 1 for inverted)
- gpio-controller : Marks the device node as a GPIO 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.
      Valid combinations are 1, 2, 3, 4, 8.
- interrupt-controller : Marks the device node as an interrupt controller.

Example:

gpio: gpio@6000d000 {
	compatible = "nvidia,tegra20-gpio";
	reg = < 0x6000d000 0x1000 >;
	interrupts = < 0 32 0x04
		       0 33 0x04
		       0 34 0x04
		       0 35 0x04
		       0 55 0x04
		       0 87 0x04
		       0 89 0x04 >;
	#gpio-cells = <2>;
	gpio-controller;
	#interrupt-cells = <2>;
	interrupt-controller;
};