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

Add support of identification registers to STM32 ADC.

By default the ADC hardware instance number is retrieved from
the compatible configuration data. Get the available ADC number
per ADC block, from hardware configuration register,
when this register exists.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20220915135452.1712453-1-olivier.moysan@foss.st.com


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