iio: magnetometer: add a driver for Voltafield AF8133J magnetometer

AF8133J is a simple I2C-connected magnetometer, without interrupts.

Add a simple IIO driver for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Dalton Durst <dalton@ubports.com>
Signed-off-by: Shoji Keita <awaittrot@shjk.jp>
Co-developed-by: Ondrej Jirman <megi@xff.cz>
Signed-off-by: Ondrej Jirman <megi@xff.cz>
Reviewed-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Tested-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Link: https://lore.kernel.org/r/20240222011341.3232645-4-megi@xff.cz
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>