iio: adc: stm32-dfsdm: manage data resolution in trigger mode

Add output sample resolution management in scan mode.
Add stm32_dfsdm_process_data() function to share sample
processing between continuous and trigger modes.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>