dt-bindings: iio: adc: Add support for MT7986

Add compatible string and specific clock property for mt7986.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Link: https://lore.kernel.org/r/20230421132047.42166-3-linux@fw-web.de


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