ASoC: dt-bindings: fsl,sai: Add AUDMIX mode support on i.MX952

ASoC: dt-bindings: fsl,sai: Add AUDMIX mode support on i.MX952

The SAI can connect to AUDMIX, but AUDMIX can be bypassed or not on
i.MX952. There are three use cases:

1) SAI -> Codec (No AUDMIX between SAI and Codec)
2) SAI -> Codec (Has AUDMIX, but AUDMIX is bypassed)
3) SAI -> AUDMIX -> Codec (Has AUDMIX and used)

So add 'fsl,sai-amix-mode' property for this feature

fsl,sai-amix-mode = "none": is for case 1)
fsl,sai-amix-mode = "bypass": is for case 2)
fsl,sai-amix-mode = "audmix": is for case 3)

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://patch.msgid.link/20260123082501.4050296-3-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>