dt-bindings: iio: dac: ti-dac082s085: Document new driver

Add device tree bindings for Texas Instruments 8/10/12-bit 2/4-channel
DAC driver.

Cc: Mathias Duckeck <m.duckeck@kunbus.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>