staging: iio: ad2s1200: Move driver out of staging

Move the iio driver for the ad2s1200 and ad2s1205 resolver-to-digital
converter out of staging, into mainline iio subsystems.

Signed-off-by: David Veenstra <davidjulianveenstra@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>