can: ctucanfd: Let users select instead of depend on CAN_CTUCANFD

The CTU CAN-FD IP core is only useful when used with one of the
corresponding PCI/PCIe or platform (FPGA, SoC) drivers, which depend on
PCI resp. OF.

Hence make the users select the core driver code, instead of letting
then depend on it.  Keep the core code config option visible when
compile-testing, to maintain compile-coverage.

Link: https://lore.kernel.org/all/887b7440446b6244a20a503cc6e8dc9258846706.1652104941.git.geert+renesas@glider.be


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>