iio: adc: ad7768-1: Add channel labels.

For better identification of the channels.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20200928090959.88842-3-cristian.pop@analog.com


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