dt-bindings:iio:adc:adc.txt: Add documentation for channel label attribute

Optional attribute for better identification of the channels.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200928090959.88842-5-cristian.pop@analog.com


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