dt-bindings: arm: fsl: add mu binding doc

The Messaging Unit module enables two processors within
the SoC to communicate and coordinate by passing messages
(e.g. data, status and control) through the MU interface.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/fsl,mu.txt

+NXP i.MX Messaging Unit (MU)
+--------------------------------------------------------------------
+
+The Messaging Unit module enables two processors within the SoC to
+communicate and coordinate by passing messages (e.g. data, status
+and control) through the MU interface. The MU also provides the ability
+for one processor to signal the other processor using interrupts.
+
+Because the MU manages the messaging between processors, the MU uses
+different clocks (from each side of the different peripheral buses).
+Therefore, the MU must synchronize the accesses from one side to the
+other. The MU accomplishes synchronization using two sets of matching
+registers (Processor A-facing, Processor B-facing).
+
+Messaging Unit Device Node:
+=============================
+
+Required properties:
+-------------------
+- compatible :	should be "fsl,<chip>-mu", the supported chips include
+		imx8qxp, imx8qm.
+- reg :		Should contain the registers location and length
+- interrupts :	Interrupt number. The interrupt specifier format depends
+		on the interrupt controller parent.
+- #mbox-cells:  Must be 0. Number of cells in a mailbox
+
+Examples:
+--------
+lsio_mu0: mailbox@5d1b0000 {
+	compatible = "fsl,imx8qxp-mu";
+	reg = <0x0 0x5d1b0000 0x0 0x10000>;
+	interrupts = <GIC_SPI 176 IRQ_TYPE_LEVEL_HIGH>;
+	#mbox-cells = <0>;
+};