staging:iio:dac:ad5446: Add support for the AD5662

The AD5662 is compatible to the AD5660, but uses an external reference instead
of an internal.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>