iio: adc: stm32-dfsdm: add fast mode support

The use of fast mode allows to get a larger set of solution
for filter parameters. This can be useful to reach a better
output sample resolution, when fast mode can be used.

Fast mode is selected at startup if it is relevant.
The startup is performed in postenable callback context,
where there are too tight time constraints for filter parameters
computation. For this reason both fast and non fast filter parameters
are pre-computed previously.

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>