dt-bindings: iio: light: add vcnl4000 devicetree bindings

Document the vishay VCNL4000 devicetree bindings.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>