arm64: dts: imx8mm: specify #sound-dai-cells for SAI nodes

Add #sound-dai-cells properties to SAI nodes.

Signed-off-by: Matt Porter <mporter@konsulko.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>