Input: atmel_mxt_ts - support 12bit resolution

Atmel touchscreen chip can support 12bit resolution and this patch
modifies to get maximum x and y size from platform data.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Iiro Valkonen <iiro.valkonen@atmel.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>