bfin_can: introduce ioremap to comply to archs with MMU

Blackfin was built without MMU, old driver code access the IO space by
physical address, introduce the ioremap approach to be compitable with
the common style supporting MMU enabled arch.

Signed-off-by: Aaron Wu <Aaron.wu@analog.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>