bindings: fsl: document compatibles of lx2160a boards

The lx2160a board compatibles "fsl,lx2160a-qds" and "fsl,lx2160a-rdb"
have been in use.  Let's document them.
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -389,6 +389,13 @@ properties:
               - fsl,ls2088a-rdb
           - const: fsl,ls2088a
 
+      - description: LX2160A based Boards
+        items:
+          - enum:
+              - fsl,lx2160a-qds
+              - fsl,lx2160a-rdb
+          - const: fsl,lx2160a
+
       - description: S32V234 based Boards
         items:
           - enum: