ACPICA: Add signatures for undefined tables: ATKG, GSCI, IEIT

These ACPI tables have been seen in the field, but the actual
table definitions are unkown at this time.

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>