arch: nrf5x: Define additional compiler macros

The current nordic hal layer in ext/ requires additional macros to
include the correct files. This will be corrected in an upcoming version
of the hal layer itself but for now add the required macros so that
users can benefit from the hal peripheral code.

Change-Id: I95127d0a98d7e882fdc779f961718fa223d504eb
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>