Documentation: DT: Add entry for Freescale LS1043a-RDB board

Signed-off-by: Mingkai Hu <[e-mail]>
Signed-off-by: Hou Zhiqiang <[e-mail]>
Signed-off-by: Li Yang <[e-mail]>
Signed-off-by: Olof Johansson <[e-mail]>

Documentation/devicetree/bindings/arm/fsl.txt

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 Freescale ARMv8 based Layerscape SoC family Device Tree Bindings
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+LS1043A ARMv8 based RDB Board
+Required root node properties:
+    - compatible = "fsl,ls1043a-rdb", "fsl,ls1043a";
+
 LS2080A ARMv8 based Simulator model
 Required root node properties:
     - compatible = "fsl,ls2080a-simu", "fsl,ls2080a";