docs: networking: device drivers: can: add flexcan

Add initial documentation for Flexcan driver.

Link: https://lore.kernel.org/all/20220107193105.1699523-8-mkl@pengutronix.de


Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>