arm: cortex_m: assert if DebugMonitor exc is enabled in debug mode

Assert if the null pointer de-referencing detection (via DWT) is
enabled when the processor is in debug mode, because the debug
monitor exception can not be triggered in debug mode (i.e. the
behavior is unpredictable). Add a note in the Kconfig definition
of the null-pointer detection implementation via DWT, stressing
that the solution requires the core be in normal mode.

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