dt-bindings:iio:adc: convert adc.txt to yaml

Each driver that uses this will need to use a $ref
We can't always enable it like most of the generic bindings due to
channel@X matching far more widely than IIO.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031182423.742798-2-jic23@kernel.org