commit 18eec2cd upstream.

Adding support for the Microsoft Surface Pro Power Cover.
Signed-off-by: Raimund Roth <raimundmroth@gmail.gom>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>