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.net


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>