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 <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml

···
   touchscreen-y-mm:
     description: vertical length in mm of the touchscreen
 
+  touchscreen-x-plate-ohms:
+    description: Resistance of the X-plate in Ohms
+
+  touchscreen-y-plate-ohms:
+    description: Resistance of the Y-plate in Ohms
+
 dependencies:
   touchscreen-size-x: [ touchscreen-size-y ]
   touchscreen-size-y: [ touchscreen-size-x ]