1Raspberry Pi GPIO expander
 2
 3The Raspberry Pi 3 GPIO expander is controlled by the VC4 firmware. The
 4firmware exposes a mailbox interface that allows the ARM core to control the
 5GPIO lines on the expander.
 6
 7The Raspberry Pi GPIO expander node must be a child node of the Raspberry Pi
 8firmware node.
 9
10Required properties:
11
12- compatible : Should be "raspberrypi,firmware-gpio"
13- gpio-controller : Marks the device node as a gpio controller
14- #gpio-cells : Should be two.  The first cell is the pin number, and
15  the second cell is used to specify the gpio polarity:
16  0 = active high
17  1 = active low
18
19Example:
20
21firmware: firmware-rpi {
22	compatible = "raspberrypi,bcm2835-firmware";
23	mboxes = <&mailbox>;
24
25	expgpio: gpio {
26		 compatible = "raspberrypi,firmware-gpio";
27		 gpio-controller;
28		 #gpio-cells = <2>;
29	 };
30};