Revert "dt-bindings: iio: magnetometer: asahi-kasei,ak8975: Drop deprecated enums"

Reverted as Rob Herring is tracking undocumented compatibles and as per
discussion in the thread, we can not remove this deprecated compatibles from
the driver without potential/likely regressions.  So keep it around
as it's not that painful anyway.

This reverts commit 711fb79a.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>