dt-bindings: fsl-imx-sdma: Add i.MX8MM/i.MX8MN/i.MX8MP compatible string

Add imx8mm/imx8mn/imx8mp sdma support.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578893602-14395-1-git-send-email-Anson.Huang@nxp.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

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

@@ -10,6 +10,9 @@ Required properties:
       "fsl,imx6q-sdma"
       "fsl,imx7d-sdma"
       "fsl,imx8mq-sdma"
+      "fsl,imx8mm-sdma"
+      "fsl,imx8mn-sdma"
+      "fsl,imx8mp-sdma"
   The -to variants should be preferred since they allow to determine the
   correct ROM script addresses needed for the driver to work without additional
   firmware.