dt-bindings: mailbox: imx-mu: add i.MX8ULP S400 MU support

Similar to i.MX8QM/QXP SCU, i.MX8ULP SCU MU is dedicated for
communication between S400 and Cortex-A cores from hardware design,
it could not be reused for other purpose. To use S400 MU more
effectivly, add "fsl,imx8ulp-mu-s4" compatile to support fast IPC.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>