IIO: ADC: add sigma delta modulator support

Add generic driver to support sigma delta modulators.
Typically, this device is hardware connected to
an IIO device in charge of the conversion. Devices are
bonded through the hardware consumer API.

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