Input: atmel_mxt_ts - split out touchpad initialisation logic

If the "linux,gpio-keymap" DT property is defined, the T19 keys are
configured and the device is setup as a touchpad rather than a touchscreen.
The logic is part of the input device initialization routine but it can be
factored out to its own function to simplify the former.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>