Add the elan trackpad to the Acer C720 (peppy) list, as it is an alternate
trackpad option. It may exist at i2c address 0x15.

Based on this change from the chromeos kernel :
https://chromium-review.googlesource.com/186253Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>