iio: adc: stm32-dfsdm: add scaling support to dfsdm

Add scaling support to STM32 DFSDM.

When used in an analog context, a DFSDM filter typically converts the data
from a sigma delta modulator. The IIO device associated to the DFSDM
filter provides these data as raw data.
The IIO device can provide scaling information (voltage and offset) to
allow conversion of raw data into physical values.

With the new binding based on IIO backend framework, the sigma delta
modulators are defined as backends providing scaling information.

The scaling is not supported with legacy binding.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Acked-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240730084640.1307938-10-olivier.moysan@foss.st.com


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