Input: elantech - add v4 hardware support

v4 hardware is a true multitouch capable touchpad (up to 5 fingers).
The packet format is quite complex, please see protocol document for
reference.

Signed-off-by: JJ Ding <jj_ding@emc.com.tw>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>