dt-bindings: input: touchscreen: msg2638: Document keys support

Document optional linux,keycodes support.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NVincent Knecht <vincent.knecht@mailoo.org>
Link: https://lore.kernel.org/r/20221116211622.2155747-2-vincent.knecht@mailoo.orgSigned-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/mstar,msg2638.yaml

@@ -36,6 +36,10 @@ properties:
   touchscreen-size-x: true
   touchscreen-size-y: true
 
+  linux,keycodes:
+    minItems: 1
+    maxItems: 4
+
 additionalProperties: false
 
 required: