dma: imx-sdma: add i.mx6ul compatible name

Add i.mx6ul compatible name in binding doc.
Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/dma/fsl-imx-sdma.txt

@@ -9,6 +9,7 @@ Required properties:
       "fsl,imx53-sdma"
       "fsl,imx6q-sdma"
       "fsl,imx7d-sdma"
+      "fsl,imx6ul-sdma"
       "fsl,imx8mq-sdma"
       "fsl,imx8mm-sdma"
       "fsl,imx8mn-sdma"