Input: of_touchscreen - add support for touchscreen-min-x|y

Some touchscreens, depending on the firmware and/or the digitizer, report
coordinates which never reach 0 along one or both of their axis.

This has been seen for example on the Silead touchscreens on a Onda V891w
and a Point of View mobii TAB-P800w(v2.0).

This commit adds support for touchscreen-min-x and touchscreen-min-y
device-properties which can be set to communicate the actual start
coordinates (rather then 0,0) to userspace.

This commit also drop the "(in pixels)" comment from the documentation
of the touchscreen-size-x and touchscreen-size-y properties. The comment
suggested that there is a relation between the range of reported
coordinates and the display resolution, which is only true for some
devices. The "(in pixels)" comment is replaced with "(maximum x coordinate
reported + 1)" to mirror the language describing the new touchscreen-min-x
and -min-y properties.

When set this fixes e.g. not being able to click things in the GNOME3
top-bar on the 2 example tablets.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>