iio: amplifiers: hmc425a: add support for ADRF5740 Attenuator

This adds support for the Analog Devices ADRF5740 2 dB LSB, 4-Bit,
Silicon Digital Attenuator, 10 MHz to 60 GHz.
The default (maximum) gain is also set at probe time, with GPIO lines
driven high to achieve maximum gain, in contrast to other devices
where GPIOs need to be driven low.

Signed-off-by: Ana-Maria Cusco <ana-maria.cusco@analog.com>
Link: https://lore.kernel.org/r/20231113102535.51074-2-anamaria.cuscoo@gmail.com


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