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

Add i.mx6ul compatible name in binding doc.

Signed-off-by: Robin Gong <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

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

···
       "fsl,imx53-sdma"
       "fsl,imx6q-sdma"
       "fsl,imx7d-sdma"
+      "fsl,imx6ul-sdma"
       "fsl,imx8mq-sdma"
       "fsl,imx8mm-sdma"
       "fsl,imx8mn-sdma"