staging:iio:ad2s90: Move out of staging

Move ad2s90 resolver driver out of staging to the main tree.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Victor Colombo <victorcolombo@gmail.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>