iio: adc: ad4170-4: Add support for weigh scale and RTD sensors

The AD4170-4 design has features to aid interfacing with weigh scale and
RTD sensors that are expected to be setup with external circuitry for
proper sensor operation. A key characteristic of those sensors is that the
circuit they are in must be excited with a pair of signals. The external
circuit can be excited either by voltage supply or by AD4170-4 excitation
signals. The sensor can then be read through a different pair of lines
that are connected to AD4170-4 ADC.

Configure AD4170-4 to handle external circuit sensors.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/0b966dd44a977e7af004065937399e6b6e9afac7.1751289747.git.marcelo.schmitt@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: machschmitt