Brcmstb mailbox

The Brcmstb mailbox is used as the companion mailbox driver for SCMI
communications.

Refer to ./mailbox.txt for generic information about mailbox device-tree
bindings.

Required properties:

 - compatible: should be one of the following.
   - "brcm,brcmstb-mbox"
 - interrupts: The interrupt number to the for the SCMI A2P channel
	and optionally the interrupt number for the P2A channel.
 - #mbox-cells: Common mailbox binding property to identify the number
	of cells required for the mailbox specifier. Should be 1.

Example:
--------

mbox: mbox@ff6b0000 {
	compatible = "brcm,brcmstb-mbox";
	interrupts = <GIC_SGI 13 IRQ_TYPE_LEVEL_HIGH>,
		     <GIC_SGI 11 IRQ_TYPE_LEVEL_HIGH>;
	#mbox-cells = <1>;
};
