Input: add driver for Elan I2C/SMbus touchpad

This driver supports Elan I2C/SMbus touchpads found in some laptops and
also in many Chromebooks.

Signed-off-by: Duson Lin <dusonlin@emc.com.tw>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>