dt-bindings: proximity: provide vcnl3020 device tree binding document

Mostly standard i2c driver with some additional led-current option
for vcnl3020.

Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>