Input: edt-ft5x06 - document wakeup-source capability

The current driver behaviour was to enable the wakeup-source everytime.
After discussion [1] we decided to change that behaviour so the device
will act as wakeup-source only if the "wakeup-source" dt-property is
present.

The patch adds the binding documentation to enable the wakeup-source
capability.

[1] https://patchwork.kernel.org/patch/11149037/Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

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

@@ -36,6 +36,8 @@ Optional properties:
  - pinctrl-0:   a phandle pointing to the pin settings for the
                 control gpios
 
+ - wakeup-source: If present the device will act as wakeup-source
+
  - threshold:   allows setting the "click"-threshold in the range
                 from 0 to 80.