platform/chrome: chromeos_laptop - Add elan trackpad option for C720

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/186253



Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>