drivers: can: mcan: let front-end drivers supply message RAM functions

Let the Bosch M_CAN front-end drivers supply their own Message RAM
read/write/clear functions.

This is preparation for supporting per-instance Bosch M_CAN Message RAM
layouts and for accessing Bosch M_CAN IP cores over peripheral busses.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>