dt-bindings: input: atmel,maxtouch: add linux,keycodes

In some configurations the touch controller can support the touch keys.
Document the linux,keycodes property that enables those keys and
specifies the keycodes that should be used to report the key events.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: André Apitzsch <git@apitzsch.eu>
Link: https://lore.kernel.org/r/20230407-atmel_keys-v2-1-92446a4343cb@apitzsch.euSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/atmel,maxtouch.yaml

@@ -14,6 +14,9 @@ description: |
   Atmel maXTouch touchscreen or touchpads such as the mXT244
   and similar devices.
 
+allOf:
+  - $ref: input.yaml#
+
 properties:
   compatible:
     const: atmel,maxtouch
@@ -60,6 +63,10 @@ properties:
       or experiment to determine which bit corresponds to which input. Use
       KEY_RESERVED for unused padding values.
 
+  linux,keycodes:
+    minItems: 1
+    maxItems: 8
+
   atmel,wakeup-method:
     $ref: /schemas/types.yaml#/definitions/uint32
     description: |