dt: mimxrt798s/cm33_cpu0: Instantiate MU4

Instantiate the MU4 peripheral in SoC's DT. Enable it in board's DT.
Switch variant of the fsl_mu driver included.

The MU4 peripheral is used for IPC with the mimxrt798s/hifi4 domain.

Signed-off-by: Vit Stanicek <vit.stanicek@nxp.com>