Input: atmel_mxt_ts - switch from OF to generic device properties

Instead of using OF-specific APIs to fecth device properties, let's switch
to generic device properties API. This will allow us to use device
properties on legacy ChromeOS devices and get rid of platform data down
the road.

Acked-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Benson Leung <bleung@chromium.org>