iio: adc: ad4170-4: Add support for weigh scale, thermocouple, and RTD sens

The AD4170-4 design provides features to aid interfacing with weigh scales,
thermocouples, and RTD sensors, which are set up with additional circuitry
for proper sensor operation. A key characteristic of those sensors is that
the circuit they are in must be excited with a single, a pair, or two pairs
of signals. The external circuit can be excited either by a voltage supply
or by AD4170-4 excitation signals. The sensor can then be read through a
different pair of lines that are connected to the AD4170-4 ADC.

Extend the ad4170-4 driver to handle external circuit sensors.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/52686943040ecad34cc89833d4d5d37f1a51f412.1751895245.git.marcelo.schmitt@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: machschmitt