dt-bindings: iio: magnetometer: Add ak09118

Document asahi-kasei,ak09918 compatible as a fallback compatible,
ak09918 is register compatible with ak09912.

Signed-off-by: Danila Tikhonov <danila@jiaxyga.com>
Signed-off-by: Barnabás Czémán <barnabas.czeman@mainlining.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240819-ak09918-v4-3-f0734d14cfb9@mainlining.org


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