dt-bindings: soc: Add a binding for the Broadcom VCHIQ services. (v3)

Broadcom VCHIQ firmware services

Required properties:

- compatible:	Should be "brcm,bcm2835-vchiq"

- reg:		Physical base address and length of the doorbell register pair

- interrupts:	The interrupt number

		  See bindings/interrupt-controller/brcm,bcm2835-armctrl-ic.txt

Example:

mailbox@7e00b840 {

	compatible = "brcm,bcm2835-vchiq";

	reg = <0x7e00b840 0xf>;

	interrupts = <0 2>;

};