arm: cortex_m: ensure DebugMonitor is targeting Secure domain

By design, the DebugMonitor exception is only employed
for null-pointer dereferencing detection, and enabling
that feature is not supported in Non-Secure builds. So
when enabling the DebugMonitor exception, assert that
it is not targeting the Non Secure domain.

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