dt-bindings: input: touchscreen: tsc2007: Document 'wakeup-source'

dt-bindings: input: touchscreen: tsc2007: Document 'wakeup-source'

The 'wakeup-source' property is used by many devicetree files and is
also supported by the tsc2007_core driver.

Document it to avoid the following dt-schema warning:

'wakeup-source' does not match any of the regexes: '^pinctrl-[0-9]+$'

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250822213245.125901-1-festevam@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>