soc: arm: sam3x: minor fixes in soc.h and arm_mpu_regions.c

Force soc.h to include the headers generated automatically
by DTS. In addition, force arm_mpu_regions.c to include
misc/slist.h for ARRAY_SIZE() macro. arm_mpu_regions.c
does not need to include soc.h (it is included indirectly
via arm_mpu.h)

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