ASoC: fsl_sai: Add AUDMIX mode support on i.MX952

ASoC: fsl_sai: Add AUDMIX mode support on i.MX952

One of SAI interfaces is connected to AUDMIX in the i.MX952 chip, but
AUDMIX can be bypassed or not bypassed on the i.MX952 platform.

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/20260123082501.4050296-5-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>