iio: dac: ad5766: add driver support for AD5766

The AD5766/AD5767 are 16-channel, 16-bit/12-bit, voltage output dense DACs
Digital-to-Analog converters.

This change adds support for these DACs.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20210115112105.58652-3-cristian.pop@analog.com


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