dt-bindings:iio:dac:ti,dac7311 yaml conversion

Very simple conversion of this binding from txt to yaml.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Link: https://lore.kernel.org/r/20201031134110.724233-14-jic23@kernel.org