arm64: dts: imx8mq: add missing SAI nodes

Currently only SAI2 is present in the DT. Add all the other SAI
instances present on the SoC.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>