ACPICA: Permanently set _REV to the value '2'.

ACPICA commit 7af00219681ab35ebe57d64a9189fd04a5a026d8

Windows uses a value of 2, and has no plans to ever change this. So, _REV is
essentially useless for its primary purpose.

Worse, some BIOS vendors have used the difference in _REV values between
Windows and ACPICA to indicate which one is running.

It has been decided by the ACPI community to deprecate this method, and return
1 for ACPI 1.0 (32-bit integers) and 2 for ACPI 2.0 and greater (both 32-bit
and 64-bit integers.

ACPICA is changed to reflect this.

Link: https://github.com/acpica/acpica/commit/7af00219
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>