dt-bindings: input: zinitix: Document touch-keys support

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: Rob Herring <robh@kernel.org>
Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Link: https://lore.kernel.org/r/20240717-zinitix-tkey-v5-1-52ea4cd4bd50@trvn.ruSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/zinitix,bt400.yaml

@@ -16,6 +16,7 @@ maintainers:
 
 allOf:
   - $ref: touchscreen.yaml#
+  - $ref: ../input.yaml#
 
 properties:
   $nodename:
@@ -79,6 +80,15 @@ properties:
     $ref: /schemas/types.yaml#/definitions/uint32
     enum: [1, 2]
 
+  linux,keycodes:
+    description:
+      This property specifies an array of keycodes assigned to the
+      touch-keys that can be present in some touchscreen configurations.
+      If the touch-keys are enabled, controller firmware will assign some
+      touch sense lines to those keys.
+    minItems: 1
+    maxItems: 8
+
   touchscreen-size-x: true
   touchscreen-size-y: true
   touchscreen-fuzz-x: true