dt-bindings: input: matrix_keypad - add wakeup-source property

The driver recognizes standard "wakeup-source" property and there are
DTS files using it. Add the property to the binding.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502280105.REZ29MVg-lkp@intel.com/


Reviewed-by: Manuel Traut <manuel.traut@mt.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/Z8EMI9ALqYY72VBV@google.com


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