iio: vcnl4000: Export near level property for proximity sensor

When an object can be considered close to the sensor is hardware
dependent. Allowing to configure the property via device tree
allows to configure this device specific value.

This is useful for e.g. iio-sensor-proxy to indicate to userspace
if an object is close to the sensor.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>