staging:iio:accel:sca3000 use a 'fake' channel to handle freefall event registration.

This is an approach used in some newer drivers as it exposes the
compound channel events to the core rather than hiding their control
in sysfs attributes entirely via the driver.

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