mailbox: imx: support i.MX8ULP S4 MU

Like i.MX8 SCU, i.MX8ULP S4 also has vendor specific protocol.
 - bind SCU/S4 MU part to share one tx/rx/init API to make code simple.
 - S4 msg max size is very large, so alloc the space at driver probe,
   not use local on stack variable.
 - S4 MU has 8 TR and 4 RR which is different with i.MX8 MU, so adapt
   code to reflect this.

   Tested on i.MX8MP, i.MX8ULP

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