arch: arm: aarch32: cortex_m: nvic: make header self-contained

The header contains macros that make use of the Devicetree API, however,
<devicetree.h> is not included. This was "mitigated" by most <soc.h>
including <devicetree.h>.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>