ACPI / video: move video_extension.txt to Documentation/acpi

ACPI video driver is written according to ACPI spec, appendix B: Video
Extensions. So it better be put under the acpi directory instead of the
power directory. This patch moves the file there without any other
change.
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>