iio: Move ad7476 driver out of staging

The ad7476 driver is a driver for simple single channel ADCs. The driver does
not export any experimental or custom ABI files nor do the static code check
tools report any issues, so move the driver out of staging.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>