Input: of_touchscreen - register multitouch axes

So far, the DT parsing code was only setting up the regular input axes,
completely ignoring their multitouch counter parts.

Fill them with the same parameters than the regular axes.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>