staging:iio: Add event monitor example application

Add a small evtest like application to monitor events generated by an IIO
device. The application can be used as an example on how to listen for IIO
events and also is usful for testing and debugging device drivers which
generate IIO events.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>