dt-bindings:iio:light:st,uvis25: txt to yaml conversion for this UV sensor

For the example, node name is uv-sensor because the standard option
of light-sensor seemed a little too generic for this.

This one could have just been moved to trivial-devices.yaml but for now
I have kept it as a separate doc.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Link: https://lore.kernel.org/r/20201031184854.745828-21-jic23@kernel.org