iio: dac: AD8801: add Analog Devices AD8801/AD8803 support

Add support for Analog Devices AD8801/AD8803, 8 channels 8bits, Digital to
Analog converters.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>