can: m_can: Kconfig: convert the into menu

Since there is more than one base driver for the m_can core, let's
convert this into a menu.

Link: https://lore.kernel.org/r/20201130133713.269256-4-mkl@pengutronix.de


Reviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>