soc: arm: do not include kernel headers in soc.h

We shall not include core kernel headers in soc.h
header of ARM SoCs. We should try to only include
the vendor headers and auto-generated board header
from DTS. This commit implements this policy for
the SoCs, whose builds have shown to fail due to
header inclusion cycles.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>