can: sja1000: fuse of_platform into platform

The OpenFirmware probe can be merged into the standard platform
probe to leverage common code.

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>