ACPICA: Add global pointer for FACS table to simplify FACS access

Use a global pointer instead of using AcpiGetTableByIndex for
each FACS access. This simplifies the code for the Global Lock
and the Firmware Waking Vector(s).

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>