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>