[ACPI] properly detect pmtimer on ASUS a8v motherboard

Handle FADT 2.0 xpmtmr address 0 case.

http://bugzilla.kernel.org/show_bug.cgi?id=5283



Signed-off-by: Shaohua <Li&lt;shaohua.li@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>