staging:iio:dac:ad5064: Add AD5628/AD5648/AD5668 support

The AD5628/AD5648/AD5668 are similar to the AD5024/AD5044/AD5064. The difference
being that they have an internal reference voltage and 8 instead of 4 DAC
channels.

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>
2 files changed