Add support for IIO event

[lblim]

Task Description

Please add IIO event support into IIO library as some sensors is supported via IIO event.

[lblim]

This is needed for SX9500 proximity sensor support.

[barbieri]

@yongli3 can you handle this as well (or delegate to a team member)? @edersondisouza can handle the review (he is currently allocated to a different task, would take some time to get to it)

[yongli3]

@barbieri No problem! Let us focus on this one and #1998, and refactor the nodes.c as well

[lblim]

May I know whether this will be included in v1 or v2?

[barbieri]

v2, since v1 features are frozen so we can get a stable release. But it should be easy to do, so should get in the repo soon.

Also, v2 is not expected to take lots of months, likely 2-3 months to get the next stable release. Depends on the bandwidth and number of changes :-)

[lblim]

can this be tagged for v2 milestone?

[yongli3]

Reference code http://lxr.free-electrons.com/source/tools/iio/iio_event_monitor.c

[yongli3]

After discussed with @laykuanloon, this is low priority, we will focus on #2205 first