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 <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Reviewed-by: Manuel Traut <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/devicetree/bindings/input/gpio-matrix-keypad.yaml

···
       Drive inactive columns during scan,
       default is to turn inactive columns into inputs.
 
+  wakeup-source: true
+
 required:
   - compatible
   - row-gpios
···
                         0x0001006A
                         0x0101001C
                         0x0201006C>;
+
+        wakeup-source;
     };