iio: Move callback buffer to its own module

Currently the IIO callback buffer implementation is directly built into the
IIO core module when enabled. Given that the callback buffer module is
standalone functionallity there is really no reason to do this. So move it
to its own module.

Also rename the source to follow the standard IIO module naming convention
as well as add a license notice to the file.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>