dt-bindings: input: atmel_mxt_ts: Document atmel,wakeup-method and WAKE line GPIO

Some Atmel touchscreen controllers have a WAKE line that needs to be
asserted low in order to wake up controller from a deep sleep. Document
the wakeup methods and the new GPIO properties.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20210302102158.10533-2-digetx@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>