iio: dac: add support for ltc2688

The LTC2688 is a 16 channel, 16 bit, +-15V DAC with an integrated
precision reference. It is guaranteed monotonic and has built in
rail-to-rail output buffers that can source or sink up to 20 mA.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220225130129.69-2-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>