iio: magnetometer: ak8974: support AMI306 variant

Add support for AMI306 magnetometer - very similar to AMI305.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>