Digi I/O Expander GPIO

Required properties:
- compatible : Should be "digi,mca-ioexp-gpio"

- #gpio-cells : Should be two.
  - first cell is the pin number
  - second cell is used to specify flags. Flags are currently unused.

- gpio-controller : Marks the device node as a GPIO controller.

Example:

	mca_ioexp: mca_io@6e {
		gpio {
			compatible = "digi,mca-ioexp-gpio";
			gpio-controller;
			#gpio-cells = <2>;

			interrupt-parent = <&mca_ioexp>;
			interrupt-controller;
			#interrupt-cells = <2>;
		};
	};
