Input: of_touchscreen - rework the DT parsing function

The DT parsing function currently duplicates a lot of the code to parse the
touchscreen DT properties. In order to ease further additions to this
parsing routine, rework it slightly to create new helper functions.

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