Documentation: DT: Add entry for QorIQ LS1046A-RDB board

Signed-off-by: Shaohui Xie <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

Documentation/devicetree/bindings/arm/fsl.txt

···
 Required root node properties:
     - compatible = "fsl,ls1046a";
 
+LS1046A ARMv8 based RDB Board
+Required root node properties:
+    - compatible = "fsl,ls1046a-rdb", "fsl,ls1046a";
+
 LS2080A SoC
 Required root node properties:
     - compatible = "fsl,ls2080a";