acpitimer(4): add acpitimer_delay(), another delay(9) implementation

acpitimer(4): add acpitimer_delay(), another delay(9) implementation

When lapic_delay() is removed from the tree in the near future, older
machines without a constant/invariant TSC will need a delay(9)
implementation better than i8254_delay().

This patch adds acpitimer_delay(), a delay(9) implementation based on
the ACPI PM timer.  It is only preferable to i8254_delay(), so set its
quality to 1000.

With input from jsg@.

Link: https://marc.info/?l=openbsd-tech&m=166053729104923&w=2
ok mlarkin@ jsg@