dmtimer(4): switch to clockintr

dmtimer(4): switch to clockintr

- Strip out custom hardclock/statclock scheduling code.
- Wire up dmtimer_intrclock.
- For convenience, add dmtimer_reset_tisr(); we do the "clear interrupt
  bits" dance in multiple places, may as well put it in a function.
- For parity with other platforms, change stathz from 128 to 100 and
  profhz from 1024 to 1000.

Testing by stuge@ and jsg@.

v1: https://marc.info/?l=openbsd-tech&m=167060320326851&w=2
v2: https://marc.info/?l=openbsd-tech&m=167340009006972&w=2

ok mlarkin@ kettenis@