dt-bindings: touchscreen: resistive-adc-touch: add support for z1 and z2 channels

For pressure calculation based on plates resistance we need some additional
properties:
- z1 and z2 channels with additional measurements between plates
- actual resistance of the touchscreen. Currently we use only
  X-resistance.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210525054634.9134-4-o.rempel@pengutronix.de


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>