can: flexcan: update Kconfig to enable coldfire

Update flexcan to support coldfire architecture.

Any new platform should use OF, but coldfire architecture is
actually not supporting devicetrees.

Link: https://lore.kernel.org/r/20210702094841.327679-4-angelo@kernel-space.org
Signed-off-by: Angelo Dureghello <angelo@kernel-space.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>