dt-bindings: input: pm8941-pwrkey: Document wakeup-source property

dt-bindings: input: pm8941-pwrkey: Document wakeup-source property

The 'resin' keys (usually connected to a volume-down button) are
generally not supposed to wake up the device from suspend, so explicitly
document a wakeup-source property to enable this wakeup behavior.

For 'pwrkey' the default stays that pressing the button does wake up the
device from suspend.

Signed-off-by: Luca Weiss <luca@lucaweiss.eu>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250909-resin-wakeup-v1-1-46159940e02b@lucaweiss.eu
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>