drivers: can: flexcan: disable self-reception and support listen-only

Add work-around for the NXP MCUxpresso SDK not exposing APIs for
setting the listen-only (LOM) bit of the FlexCAN MCR register and the
self-reception disable (SRXDIS) bit of the CTRL1 register.

These bits can only be written when the FlexCAN module is in freeze
mode. Add a set of simplified functions (not supporting errata 9595) for
entering/exiting freeze mode.

This work-around can be removed again once the NXP MCUxpresso SDK
exposes the needed functionality.

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