arm64: dts: imx8mp: Add SAI, SDMA, AudioMIX

Add all SAI nodes, SDMA2 and SDMA3 nodes, and AudioMIX node. This is
needed to get audio operational on i.MX8MP .

Acked-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mp-beacon-kit
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>