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 <[e-mail]>
Reviewed-by: Alistair Francis <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

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

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