arm64: dts: imx8mm: Add SAI nodes

i.MX8MM has 5 SAI instances with the following base
addresses according to RM.

SAI1 base address: 3001_0000h
SAI2 base address: 3002_0000h
SAI3 base address: 3003_0000h
SAI5 base address: 3005_0000h
SAI6 base address: 3006_0000h

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>