Input: edt-ft5x06 - increase allowed data range for threshold parameter

The datasheet and application note does not mention an allowed range for
the M09_REGISTER_THRESHOLD parameter. One of our customers needs to set
lower values than 20 and they seem to work just fine on EDT EP0xx0M09 with
T5x06 touch.

So, lacking a known lower limit, we increase the range for thresholds,
and set the lower limit to 0. The documentation is updated accordingly.

Signed-off-by: Schoefegger Stefan <[e-mail]>
Signed-off-by: Manfred Schlaegl <[e-mail]>
Signed-off-by: Martin Kepplinger <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

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

···
                 control gpios
 
  - threshold:   allows setting the "click"-threshold in the range
-                from 20 to 80.
+                from 0 to 80.
 
  - gain:        allows setting the sensitivity in the range from 0 to
                 31. Note that lower values indicate higher

Documentation/input/devices/edt-ft5x06.rst

···
 The driver allows configuration of the touch screen via a set of sysfs files:
 
 /sys/class/input/eventX/device/device/threshold:
-    allows setting the "click"-threshold in the range from 20 to 80.
+    allows setting the "click"-threshold in the range from 0 to 80.
 
 /sys/class/input/eventX/device/device/gain:
     allows setting the sensitivity in the range from 0 to 31. Note that

drivers/input/touchscreen/edt-ft5x06.c

···
 static EDT_ATTR(offset, S_IWUSR | S_IRUGO, WORK_REGISTER_OFFSET,
 		M09_REGISTER_OFFSET, 0, 31);
 static EDT_ATTR(threshold, S_IWUSR | S_IRUGO, WORK_REGISTER_THRESHOLD,
-		M09_REGISTER_THRESHOLD, 20, 80);
+		M09_REGISTER_THRESHOLD, 0, 80);
 static EDT_ATTR(report_rate, S_IWUSR | S_IRUGO, WORK_REGISTER_REPORT_RATE,
 		NO_REGISTER, 3, 14);