dt-bindings: gpio: add raspberry pi GPIO expander binding

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>;

	 };

};