IIO: ADC: add stm32 DFSDM core support

Add driver for stm32 DFSDM pheripheral. Its converts a sigma delta
stream in n bit samples through a low pass filter and an integrator.
stm32-dfsdm-core driver is the core part supporting the filter
instances dedicated to sigma-delta ADC or audio PDM microphone purpose.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>