IIO: inkern: API for manipulating channel attributes

Extend the inkern API with functions for reading and writing
attribute of iio channels.

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>
4 files changed