dt-bindings: mms114: Add linux,keycodes property for touch keys

MELFAS MMS114 and similar touchscreens have support for touch keys.
Add the linux,keycodes property which can be used to set up the
keycodes for the touch keys.
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Artur Weber <aweber.kernel@gmail.com>
Link: https://lore.kernel.org/r/20230714100424.29798-2-aweber.kernel@gmail.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/melfas,mms114.yaml

@@ -52,6 +52,11 @@ properties:
   touchscreen-swapped-x-y: true
   touchscreen-max-pressure: true
 
+  linux,keycodes:
+    description: Keycodes for the touch keys
+    minItems: 1
+    maxItems: 15
+
 additionalProperties: false
 
 required: