mailbox: imx: populate sub-nodes

Some MUs such as i.MX95 MU, have internal SRAM which could be used
for SCMI shared memory, so populate the sub-nodes to use the SRAM.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>