iio:adc:ad7280a: Move out of staging

This is a rather unusual device (in IIO anyway).  However, it has
a near to standard userspace ABI.

Note the work to move this out of staging was done against a minimal
QEMU model, which doesn't model all the features of the device.
I have no intention to upstream the QEMU model as it was developed
just to enable this driver cleanup.

https://github.com/jic23/qemu/tree/ad7280a-hacks



Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-21-jic23@kernel.org