IIO: add DT bindings for stm32 DFSDM filter

Add bindings that describes STM32 Digital Filter for Sigma Delta
Modulators. DFSDM allows to connect sigma delta
modulators.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>