arm: set DebugMonitor IRQ unconditionally during initialization

If the DebugMonitor extension is implemented by the core,
the interrupt may be pended and become active, even if it
is not enabled. Set the priority level of DebugMonitor upon
system initialization to the intended value unconditionally
so we do not end up in undefined behavior, if the exception
is accidentally pended. Since the priority level is set at
init, we can remove resetting the priority in DWT driver
initialization.

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