iio:ad5446: Add support for the ad5300/ad5310/ad5320

The ad5300/ad5310/ad5320 is a family of single channel DACs with a SPI interface
similar to the ad5601/ad5611/ad5621 but use a different shift factor for the
data word.

While we are at it also reorder the device part numbers in the ad5446 driver
Kconfig to be ordered alphabetically.

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