iio: adc: ad7173: add AD7173 driver

The AD7173 family offer a complete integrated Sigma-Delta ADC solution
which can be used in high precision, low noise single channel
applications or higher speed multiplexed applications. The Sigma-Delta
ADC is intended primarily for measurement of signals close to DC but also
delivers outstanding performance with input bandwidths out to ~10kHz.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Reviewed-by: Michael Walle <michael@walle.cc> # for gpio-regmap
Signed-off-by: Dumitru Ceclan <mitrutzceclan@gmail.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240228110622.25114-3-mitrutzceclan@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>