ACPICA: Make ACPI Power Management Timer (PM Timer) optional.

ACPICA: Make ACPI Power Management Timer (PM Timer) optional.

PM Timer is now optional.
This support is already in Windows8 and "SHOULD" come out in ACPI 5.0A
(if all goes well).

The change doesn't affect Xen directly, because it does not rely
on the presence of the PM timer.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
[ported to Xen]
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>