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 <[e-mail]>
Signed-off-by: Nikita Travkin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

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

···
 
 allOf:
   - $ref: touchscreen.yaml#
+  - $ref: ../input.yaml#
 
 properties:
   $nodename:
···
       1 and 2. Mode 2 is the default and preferred.
     $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