iio: add enable and disable services to iio backend framework

Add iio_backend_disable() and iio_backend_enable() APIs to allow
IIO backend consumer to request backend disabling and enabling.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240730084640.1307938-3-olivier.moysan@foss.st.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>