Input: goodix - refer to touchscreen.txt in device tree bindings

Refer to touchscreen.txt for generic touch properties. This avoids
duplication and we're using the generic code to parse these in the
driver. While at that add touchscreen-size-{x,y} which are respected by
the driver as well.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>