IIO: Add DT bindings for sigma delta adc modulator

Add documentation of device tree bindings to support
sigma delta modulator in IIO framework.

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>