dt-bindings:iio:dac:fsl,vf610-dac yaml conversion

Simple binding to convert. Example expanded a little to include
an example bus.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Sanchayan Maity <maitysanchayan@gmail.com>
Link: https://lore.kernel.org/r/20201031134110.724233-20-jic23@kernel.org