commit b90b3c4a upstream.

Add support for the acer c720p touchscreen.
Tested manually by using the touchscreen on the acer c720p-2664

Based on the following patch by Dave Parker <dparker@chromium.org>:
https://chromium-review.googlesource.com/#/c/167136/Signed-off-by: Michael Mullin <masmullin@gmail.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Scot Doyle <lkml14@scotdoyle.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>