ACPICA: Add infrastructure for External() opcode.

ACPICA commit d115fe2ffdab449d6107d58580c5afd0a81d65fe

This change adds the basic low-level infrastructure for the External
AML opcode. The interpreter will simply ignore this op, as the op
is intended for use by the disassembler only.

Note that External() opcode is useful for disassembler, interpreter
can simply ignore it and still return exceptions for unknown control
methods so the kernel part only includes the grammar definition of
External() opcode in order to ignore it but doesn't interpret it.

Link: https://github.com/acpica/acpica/commit/d115fe2f
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>