arm64: dts: imx8mn: specify #sound-dai-cells for SAI nodes

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

Reviewed-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Fixes: 9e986006 ("arm64: dts: imx8mn: Add SAI nodes")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>