ASoC: dt-bindings: mediatek,mt7986-afe: add audio afe document

Add mt7986 audio afe document.

Signed-off-by: Maso Huang <maso.huang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230817101338.18782-7-maso.huang@mediatek.com


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