dt-bindings: touchscreen: add touchscreen-x/y-plate-ohms property

To calculate the pressure on a restrictive touchscreen we need to know
resistance at least of X plate. Some calculations need to use both X and
Y values. So let's add generic properties which can be used by all
drivers.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210525054634.9134-3-o.rempel@pengutronix.de


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