staging:iio:dac:ad5064: Add AD5666 support

The AD5666 is identical to the ad5064-1, except that it has a internal reference
voltage.

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>