hpet: Allow a MMIO window smaller than 1K

Some new AMD systems provide a HPET MMIO region smaller than the 1KB
specified, and a correspondingly small number of timers. Handle this in
the HPET driver rather than requiring a 1KB window. This allows the
HPET driver to attach on such systems.

PR: 262638
Reviewed by: markj
MFC after: 1 month