ACPICA: Remove use of unreliable FADT revision field

The revision number in the FADT has been found to be completely
unreliable and cannot be trusted. Only the table length can be
used to infer the actual version.

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>