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

The register layout and bits definition of i.MX8ULP MU is different
compared with others, let's add the compatible for the new MU.

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