powerpc64: switch to clockintr

powerpc64: switch to clockintr

- Remove powerpc64-specific clock interrupt scheduling bits from
  cpu_info.
- Remove powerpc64-specific randomized statclock() bits from
  powerpc64/clock.c.
- Remove the 'stat_count' evcount.  All clock interrupts are now counted
  via the 'clock_count' evcount.
- Wire up dec_intrclock.

Bringup help from gkoehler@.  Tested by gkoehler@: this patch has
survived four kernel-release-upgrade cycles on a Raptor Talos II
T2P9S01 sporting a quad-core POWER9 CPU.

Link: https://marc.info/?l=openbsd-tech&m=166776404803622&w=2
ok gkoehler@ mlarkin@