drivers: can: mcan: let front-end drivers supply register r/w functions

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

This is preparation for handling non-standard Bosch M_CAN register layouts
directly in the front-end and for accessing Bosch M_CAN IP cores over
peripheral busses.

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