Broadcom BCM2835 VideoCore mailbox IPC

Required properties:

- compatible:	Should be "brcm,bcm2835-mbox"
- reg:		Specifies base physical address and size of the registers
- interrupts:	The interrupt number
		  See bindings/interrupt-controller/brcm,bcm2835-armctrl-ic.txt
- #mbox-cells:	Specifies the number of cells needed to encode a mailbox
		  channel. The value shall be 0, since there is only one
		  mailbox channel implemented by the device.

Example:

mailbox: mailbox@7e00b800 {
	compatible = "brcm,bcm2835-mbox";
	reg = <0x7e00b880 0x40>;
	interrupts = <0 1>;
	#mbox-cells = <0>;
};

firmware: firmware {
	compatible = "raspberrypi,firmware";
	mboxes = <&mailbox>;
	#power-domain-cells = <1>;
};