can: add driver for Softing card

This patch adds the driver that creates a platform:softing device
from a pcmcia_device
Note: the Kconfig indicates a dependency on the softing.ko driver,
but this is purely to make configuration intuitive. This driver will
work independent, but no CAN network devices appear until softing.ko is
loaded too.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>