Currently the driver relies on some obscure and undocumented register to set
the maximum axis value.

The reported value is way too high to be meaningful, which confuses some
userspace tools like QT's evdevtouch plugin which try to scale the reported
events to the maximum values.

Use the values from the DT to set meaningful values.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>