iio: amplifiers: hmc425a: add support for LTC6373 Instrumentation Amplifier

This adds support for LTC6373 36 V Fully-Differential Programmable-Gain
Instrumentation Amplifier with 25 pA Input Bias Current.
The user can program the gain to one of seven available settings through
a 3-bit parallel interface (A2 to A0).

Signed-off-by: Dumitru Ceclan <mitrutzceclan@gmail.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240220153553.2432-6-mitrutzceclan@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>