dt-bindings: iio: adc: Add compatible for MT8188

Add dt-binding documentation of auxadc for MediaTek MT8188 SoC platform.

Signed-off-by: Hui.Liu <hui.liu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220719014657.28714-2-hui.liu@mediatek.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>