dt-bindings: iio: adc: Add compatible for Mediatek MT8186

This commit adds dt-binding documentation of auxadc for Mediatek MT8186 SoC
Platform.

Signed-off-by: Guodong Liu <guodong.liu@mediatek.com>
Acked-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220110084841.575-1-guodong.liu@mediatek.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>