dt-bindings: iio: Introduce common properties for iio sensors

Introduce a file for common properties of iio sensors. So far this
contains the new proximity-near-level property for proximity sensors
that indicates when an object should be considered near.

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