Input: ads7846 - add support for general touchscreen bindings

A few vendor specific bindings are now covered by common bindings.

Let the driver parse the common bindings to make use of common
inverting and swapping mechnism. Aslo make use of
touchscreen_report_pos() to ensure the correct inverting-swapping
order.

The vendor specific properties are used as default (backward
compatibility) and gets overwritten by common bindings.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>