iio: adc: ad7923: Add support for the ad7908/ad7918/ad7928

The ad7928 is software compatible with the ad7923.
The ad7908 and ad7918 are the 8 and 10-bit versions of the ad7928.

Signed-off-by: Daniel Junho <djunho@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>