dt-bindings: input: touchscreen: msg2638: Document msg2138 support

Document msg2138 support by adding mstar,msg2138 compatible.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NVincent Knecht <vincent.knecht@mailoo.org>
Link: https://lore.kernel.org/r/20221110171952.34207-3-vincent.knecht@mailoo.orgSigned-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>

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

@@ -14,7 +14,9 @@ allOf:
 
 properties:
   compatible:
-    const: mstar,msg2638
+    enum:
+      - mstar,msg2138
+      - mstar,msg2638
 
   reg:
     const: 0x26