staging: iio: ad7780: moving ad7780 out of staging

Move ad7780 ADC driver out of staging and into the mainline.

The ad7780 is a sigma-delta analog to digital converter. This driver provides
reading voltage values and status bits from both the ad778x and ad717x series.
Its interface also allows writing on the FILTER and GAIN GPIO pins on the
ad778x.

Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Signed-off-by: Giuliano Belinassi <giuliano.belinassi@usp.br>
Co-developed-by: Giuliano Belinassi <giuliano.belinassi@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>