staging:iio:dac:ad5064: Add AD5025/AD5045/AD5065 support

The AD5025/AD5045/AD5065 are identical to the AD5024/AD5044/AD5064 except that
they have 2 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>