ACPICA: Disassembler: Add support for Operation Region externals

Adds missing support for operation regions defined in another
table, but referenced via a Field or BankField operator. Generate
the correct External statement.

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>