dt-bindings: input: cypress,tt21000 - add wakeup-source entry to documentation

The touchscreen can be used to wake up systems from sleep and therefore
the wakeup-source entry should be included in the documentation.
Signed-off-by: Maximilian Weigand <mweigand@mweigand.net>
Reviewed-by: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230501113010.891786-6-mweigand@mweigand.netSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/cypress,tt21000.yaml

@@ -40,6 +40,8 @@ properties:
   linux,keycodes:
     description: EV_ABS specific event code generated by the axis.
 
+  wakeup-source: true
+
 patternProperties:
   "^button@[0-9]+$":
     type: object