dt-bindings: iio: dac: Add ti,dac7311 device tree bindings in documentation

Description how to invoke ti-dac7311 driver from device tree file.

Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>