[PATCH] x86_64: fix hpet for systems that don't support legacy replacement

Currently the x86-64 HPET code assumes the entire HPET implementation from
the spec is present.  This breaks on boxes that do not implement the
optional legacy timer replacement functionality portion of the spec.

This patch fixes this issue, allowing x86-64 systems that cannot use the
HPET for the timer interrupt and RTC to still use the HPET as a time
source.  I've tested this patch on a system systems without HPET, with HPET
but without legacy timer replacement, as well as HPET with legacy timer
replacement.

This version adds a minor check to cap the HPET counter value in
gettimeoffset_hpet to avoid possible time inconsistencies.  Please ignore
the A2 version I sent to you earlier.

Acked-by: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>