docs: driver-api: add iio hw consumer section

This adds a section about the Hardware consumer
API of the IIO subsystem to the driver API
documentation.

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>