[PATCH] hpet-RTC: cache the comparator register

Reads from an HPET register require a round trip to the south bridge and are
almost as slow as PCI reads.  By caching the last value we've written to the
comparator register, we can eliminate all HPET reads from the fast path in the
emulated RTC interrupt handler.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>