iio: adc: stm32-dfsdm: add id registers support

Add support of identification registers to STM32 DFSDM
to allow hardware capabilities discovery and configuration check.
The number of filters and channels, are read from registers,
when they are available.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://lore.kernel.org/r/20230105125331.328275-1-olivier.moysan@foss.st.com


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