ACPI: Update MAINTAINERS file to include Documentation/acpi

Documentation/acpi contains all of the Docunmentation for the
Linux/ACPI subsystem.  Adds this to the MAINTAINERS file.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>