mailbox: imx: get RR/TR registers num from Parameter register

i.MX8ULP, i.MX93 MU has a Parameter register encoded as below:
BIT: 15 --- 8 | 7 --- 0
      RR_NUM    TR_NUM

So to make driver easy to support more variants, get the RR/TR
registers number from Parameter register.

The patch only adds support the specific MU, such as ELE MU.
For generic MU, not add support for number larger than 4.

Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>