ACPICA: Ignore ACPI table signature for Load() operator

Only "SSDT" is acceptable to the ACPI spec, but tables are
seen with OEMx and null sigs. Therefore, signature validation
is worthless.  Apparently MS ACPI accepts such signatures, ACPICA
must be compatible.

http://bugzilla.kernel.org/show_bug.cgi?id=10454

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>