iio: amplifiers: ad8366: add support for HMC1119 Attenuator

iio: amplifiers: ad8366: add support for HMC1119 Attenuator

This change adds support for the HMC1119 Silicon Digial Attenuator. The
HMC1119 is a broadband, highly accurate, 7-bit digital attenuator,
operating from 0.1 GHz to 6.0 GHz with 31.5 dB attenuation control range
in 0.25 dB steps.

Link: https://www.analog.com/media/en/technical-documentation/data-sheets/hmc1119.pdf
Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>