staging:iio: Move DAC drivers out of staging

The IIO DAC drivers are in a reasonably good shape. They all make use of channel
spec and non of them provides non-documented sysfs attributes. Code style should
be OK as well, both checkpatch and coccicheck only report trivial issues.

So lets move the whole folder out of staging.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>