iio: mma8452: add freefall / motion interrupt source

This adds the freefall / motion interrupt source definitions to the driver.
It is used in this series' next patch, for chips that don't support the
transient interrupt source.

Signed-off-by: Martin Kepplinger <martin.kepplinger@theobroma-systems.com>
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>