iio: amplifiers: ad8366: add support for HMC792A Attenuator

This change adds support for the HMC792A Digital Attenuator. The
HMC792A is a broadband 6-bit GaAs MMIC Digital Attenuator operating
from DC to 6.0 GHz with 15.75 dB attenuation control range in 0.25 dB steps.

Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/hmc792a.pdf
Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Link: https://lore.kernel.org/r/20230721121038.183404-1-kimseer.paller@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>