staging:iio: Factor out event handling into its own file

The core iio file has gotten quite cluttered over time. This patch moves
the event handling code into its own file. Since the event handling code is
largely independent from the core code the only code changes necessary for
this are to make the moved iio_device_register_eventset,
iio_device_unregister_eventset and iio_event_getfd functions non static.

This has also the advantage that industrialio-core.c is now closer again to
its counterpart in the outofstaging branch.

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>