dt-bindings: input: matrix_keypad - add missing property

The property is implemented in the driver but not described in dt-bindings.
Add missing property 'gpio-activelow' to DT schema.

Signed-off-by: Markus Burri <markus.burri@mt.com>
Link: https://lore.kernel.org/r/20250226152843.43932-3-markus.burri@mt.com


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