iio: dac: Kconfig: sort symbols alphabetically

While adding a new device, I noticed these aren't sorted alphabetically [as
in the Makefile], which messed my head-up.
This change sorts the devices alphabetically in the drivers/iio/dac/Kconfig
file as well.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>