dt-bindings: mediatek: mt8192: add audio afe document

This patch adds mt8192 audio afe document.

Signed-off-by: Jiaxin Yu <jiaxin.yu@mediatek.com>
Link: https://lore.kernel.org/r/1604390378-23993-8-git-send-email-jiaxin.yu@mediatek.com


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