iio: adc: stm32-adc: add debugfs to read raw calibration result

Add debugfs to read linear ADC STM32 self calibration results.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
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/20221115103124.70074-4-olivier.moysan@foss.st.com


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