arm64: dts: imx8mn: Fix SAI nodes

The most specific compatible string element should be "fsl,imx8mn-sai"
on i.MX8M Nano, fix it from current "fsl,imx8mm-sai" (two Ms, likely
due to copy-paste error from i.MX8M Mini).

Fixes: 9e986006 ("arm64: dts: imx8mn: Add SAI nodes")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Adam Ford <aford173@gmail.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: NXP Linux Team <linux-imx@nxp.com>
To: linux-arm-kernel@lists.infradead.org
Reviewed-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>