dt-bindings: mailbox: imx-mu: add SCU MU support

i.MX8/8X SCU MU is dedicated for communication between SCU
and Cortex-A cores from hardware design, it could not be reused
for other purpose. To use SCU MU more effectivly, add "fsl,imx8-scu-mu"
compatile to support fast IPC.
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

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

@@ -23,6 +23,8 @@ Required properties:
 		be included together with SoC specific compatible.
 		There is a version 1.0 MU on imx7ulp, use "fsl,imx7ulp-mu"
 		compatible to support it.
+		To communicate with i.MX8 SCU, "fsl,imx8-mu-scu" could be
+		used for fast IPC
 - reg :		Should contain the registers location and length
 - interrupts :	Interrupt number. The interrupt specifier format depends
 		on the interrupt controller parent.