drivers: can: can_nrf: add missing soc.h include

The can_nrf driver uses defines from soc.h which happened to be
included by some other header when building for cpuapp, but not
when building for cpuflpr. Include soc.h explicitly.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>