Cleanup/bugfix HPET driver:
- hpet_init() could return failure with driver still registered with ACPI
- driver add() functions were marked __init, but can be called any time
until driver is unregistered (not a real problem until HPETs can be
hotplugged)
- time interpolator registration now done in driver add() path, so we can
support removal of HPETs someday
- physical MMIO address should be printed, not ioremapped address