Input: elan_i2c - add support for ELAN0621 touchpad

Added the ability to detect the ELAN0621 touchpad found in some Lenovo
laptops.

Signed-off-by: Adam Wong <adam@adamwong.me>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>