Input: edt-ft5x06 - document wakeup-source capability

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>