dt-bindings: iio: dac: add bindings for ltc1660

LTC1665/LTC1660 is a 8/10-bit Digital-to-Analog Converter (DAC)
with eight individual channels.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>