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
   - "rockchip,rk3576-mbox" for rk3576
 - 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

Optional properties :

 - wakeup-source: Mailbox irq can be used as a wakeup source.
 - rockchip,txpoll-period-us: TX Done polling interval in microseconds,
	only support when CONFIG_MAILBOX_POLL_PERIOD_US is selected.
 - rockchip,txpoll-period-ms: TX Done polling interval in milliseconds.
 - rockchip,enable-cmd-trigger: Enable write cmd register to trigger interrupt.
	This is only support from rockchip,rk3576-mbox.
 - rockchip,txdone-ack: Enable if the mailbox client use ACK to check TX_DONE.
 - rockchip,txdone-irq: Enable if the controller can trigger TX_DONE interrupt.
 - rockchip,tx-direction-b2a: To specify the TX direction as B2A for local.

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>;
};