arm64: dts: imx8mn: fix compatible string for sdma

SDMA in i.MX8MN should use same configuration as i.MX8MQ
So need to change compatible string to be "fsl,imx8mq-sdma".

Fixes: 6c3debcb

 ("arm64: dts: freescale: Add i.MX8MN dtsi support")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>