dapmic(4): support clean shutdown with the power button

dapmic(4): support clean shutdown with the power button

Register an interrupt handler and look for nONKEY events.
Mask off all other events.  Also clear the FAULT_LOG register at
startup, but don't attempt to acknowledge its contents at runtime.  This
lets the user force a shutdown with a long power button press.  Some
events in EVENT_* or FAULT_LOG may benefit from more graceful handling.

This makes it possible to cleanly shutdown my Hifive Unmatched using
the power button.

Thanks to kettenis@ who wrote sfgpio(4) to handle dapmic(4) interrupts
on the Unmatched.  ok kettenis@