ASoC: dt-bindings: mediatek,mt7986-wm8960: add mt7986-wm8960 document

Add document for mt7986 board with wm8960.

Signed-off-by: Maso Huang <maso.huang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230817101338.18782-6-maso.huang@mediatek.com


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