dt-bindings: input: fsl,scu-key: Document wakeup-source

dt-bindings: input: fsl,scu-key: Document wakeup-source

The SCU Key controller can be used as a system wakeup source.

Document the 'wakeup-source' property.

This fixes the following schema warning:

system-controller: keys: 'wakeup-source' does not match any of the regexes: 'pinctrl-[0-9]+'
	from schema $id: http://devicetree.org/schemas/firmware/fsl,scu.yaml#

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230926122957.341094-1-festevam@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>