dt-bindings: mailbox: imx-mu: add imx7ulp MU support

There is a version 1.0 MU on imx7ulp, use "fsl,imx7ulp-mu" compatible
to support it.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>