alpha: switch to clockintr

alpha: switch to clockintr

- Add missing tick_nsec initialization to cpu_initclocks().
- Set stathz = hz, profhz = stathz; we don't have any control over the
  interrupt clock on alpha so everything has the same frequency.
- Set schedhz = 16 to imitate current schedclock() dispatch frequency.

Bringup help from claudio@.  Tested by miod@ (2-CPU DS25).

Link: https://marc.info/?l=openbsd-tech&m=166776333303245&w=2
ok mlarkin@