Avionic Design N-bit GPIO expander bindings

Required properties:
- compatible: should be "ad,gpio-adnp"
- reg: The I2C slave address for this device.
- interrupts: Interrupt specifier for the controllers interrupt.
- #gpio-cells: Should be 2. The first cell is the GPIO number and the
  second cell is used to specify optional parameters:
  - bit 0: polarity (0: normal, 1: inverted)
- gpio-controller: Marks the device as a GPIO controller
- nr-gpios: The number of pins supported by the controller.

The GPIO expander can optionally be used as an interrupt controller, in
which case it uses the default two cell specifier as described in
Documentation/devicetree/bindings/interrupt-controller/interrupts.txt.

Example:

	gpioext: gpio-controller@41 {
		compatible = "ad,gpio-adnp";
		reg = <0x41>;

		interrupt-parent = <&gpio>;
		interrupts = <160 1>;

		gpio-controller;
		#gpio-cells = <2>;

		interrupt-controller;
		#interrupt-cells = <2>;

		nr-gpios = <64>;
	};