Input: edt-ft5x06 - add offset support for ev-ft5726

Unfortunately the evervision focaltech implementation uses two offset
registers, one for the x coordinate and one for y.

This patch extends the driver to handle those offset registers only for
devices that support these.

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>

Documentation/devicetree/bindings/input/touchscreen/edt-ft5x06.txt

@@ -44,6 +44,15 @@ Optional properties:
 
  - offset:      allows setting the edge compensation in the range from
                 0 to 31.
+
+ - offset-x:    Same as offset, but applies only to the horizontal position.
+                Range from 0 to 80, only supported by evervision,ev-ft5726
+                devices.
+
+ - offset-y:    Same as offset, but applies only to the vertical position.
+                Range from 0 to 80, only supported by evervision,ev-ft5726
+                devices.
+
  - touchscreen-size-x	   : See touchscreen.txt
  - touchscreen-size-y	   : See touchscreen.txt
  - touchscreen-fuzz-x      : See touchscreen.txt