drivers: ipm: Select MBOX from IPM_MBOX

Projects using IPM will select CONFIG_IPM=y, then the appropriate driver
will be enabled based on DT. For devices using the IPM over MBOX driver
this will be config IPM_MBOX. But this config depends on MBOX so if the
project has not also enabled that, then this driver will not be selected.

To fix this, select MBOX from IPM_MBOX. This causes the correct MBOX
driver to then be selected also based on DT. This allows projects using
only IPM to only need to select the same as before, MBOX will be selected
as needed based on DT.

Signed-off-by: Andrew Davis <afd@ti.com>