ASoC: mediatek: mt8186: add audio afe document

Add mt8186 audio afe document.

Signed-off-by: Jiaxin Yu <jiaxin.yu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220718162204.26238-5-jiaxin.yu@mediatek.com


Signed-off-by: Mark Brown <broonie@kernel.org>