dt-bindings: iio: light: vcnl4000: Add proximity-near-level

This value indicates when userspace should consider an object
near to the sensor/device.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>