Raspberry Pi GPIO expander

The Raspberry Pi 3 GPIO expander is controlled by the VC4 firmware. The
firmware exposes a mailbox interface that allows the ARM core to control the
GPIO lines on the expander.

The Raspberry Pi GPIO expander node must be a child node of the Raspberry Pi
firmware node.

Required properties:

- compatible : Should be "raspberrypi,firmware-gpio"
- gpio-controller : Marks the device node as a gpio controller
- #gpio-cells : Should be two.  The first cell is the pin number, and
  the second cell is used to specify the gpio polarity:
  0 = active high
  1 = active low

Example:

firmware: firmware-rpi {
	compatible = "raspberrypi,bcm2835-firmware";
	mboxes = <&mailbox>;

	expgpio: gpio {
		 compatible = "raspberrypi,firmware-gpio";
		 gpio-controller;
		 #gpio-cells = <2>;
	 };
};