Rockchip mailbox

The Rockchip mailbox is used by the Rockchip CPU cores to communicate
requests to MCU processor.

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

Required properties:

 - compatible: should be one of the following.
   - "rockchip,rk3368-mbox" for rk3368
 - reg: physical base address of the controller and length of memory mapped
	region.
 - interrupts: The interrupt number to the cpu. The interrupt specifier format
	depends on the interrupt controller.
 - #mbox-cells: Common mailbox binding property to identify the number
	of cells required for the mailbox specifier. Should be 1

Example:
--------

/* RK3368 */
mbox: mbox@ff6b0000 {
	compatible = "rockchip,rk3368-mailbox";
	reg = <0x0 0xff6b0000 0x0 0x1000>,
	interrupts = <GIC_SPI 146 IRQ_TYPE_LEVEL_HIGH>,
		     <GIC_SPI 147 IRQ_TYPE_LEVEL_HIGH>,
		     <GIC_SPI 148 IRQ_TYPE_LEVEL_HIGH>,
		     <GIC_SPI 149 IRQ_TYPE_LEVEL_HIGH>;
	#mbox-cells = <1>;
};
