can: sja1000: of: add reg-io-width property for 8, 16 and 32-bit register access

Add the 'reg-io-width' property for 8, 16 and 32-bit access, like
what is currently done with IORESOURCE_MEM_{8,16,32}BIT for non-OF
boot.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Tested-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>