Documentation: mailbox: Add APM X-Gene SLIMpro mailbox dts documentation

The APM X-Gene SLIMpro mailbox is used to communicate messages between

the ARM64 processors and the Cortex M3 (dubbed SLIMpro). It uses a simple

interrupt based door bell mechanism and can exchange simple messages using the

internal registers.

There are total of 8 interrupts in this mailbox. Each used for an individual

door bell (or mailbox channel).

Required properties:

- compatible:	Should be as "apm,xgene-slimpro-mbox".

- reg:		Contains the mailbox register address range.

- interrupts:	8 interrupts must be from 0 to 7, interrupt 0 define the

		the interrupt for mailbox channel 0 and interrupt 1 for

		mailbox channel 1 and so likewise for the reminder.

- #mbox-cells:	only one to specify the mailbox channel number.

Example:

Mailbox Node:

		mailbox: mailbox@10540000 {

			compatible = "apm,xgene-slimpro-mbox";

			reg = <0x0 0x10540000 0x0 0xa000>;

			#mbox-cells = <1>;

			interrupts =  	<0x0 0x0 0x4>,

					<0x0 0x1 0x4>,

					<0x0 0x2 0x4>,

					<0x0 0x3 0x4>,

					<0x0 0x4 0x4>,

					<0x0 0x5 0x4>,

					<0x0 0x6 0x4>,

					<0x0 0x7 0x4>,

		};