dt-bindings: input: syna,rmi4: Document F1A function

In some configurations the touch controller can support touch keys.
Document the linux,keycodes property that enables those keys and
specifies the keycodes that should be used to report the key events.

Signed-off-by: André Apitzsch <git@apitzsch.eu>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250707-rmi4_f1a-v1-1-838d83c72e7f@apitzsch.eu
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/syna,rmi4.yaml

@@ -89,6 +89,24 @@ properties:
     required:
       - reg
 
+  rmi4-f1a@1a:
+    type: object
+    additionalProperties: false
+    $ref: input.yaml#
+    description:
+      RMI4 Function 1A is for capacitive keys.
+
+    properties:
+      reg:
+        maxItems: 1
+
+      linux,keycodes:
+        minItems: 1
+        maxItems: 4
+
+    required:
+      - reg
+
 patternProperties:
   "^rmi4-f1[12]@1[12]$":
     type: object
@@ -201,6 +219,7 @@ allOf:
 
 examples:
   - |
+    #include <dt-bindings/input/linux-event-codes.h>
     #include <dt-bindings/interrupt-controller/irq.h>
 
     i2c {
@@ -234,6 +253,7 @@ examples:
 
             rmi4-f1a@1a {
                 reg = <0x1a>;
+                linux,keycodes = <KEY_BACK KEY_HOME KEY_MENU>;
             };
         };
     };