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>

Documentation/devicetree/bindings/iio/common.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/common.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Common properties for iio sensors
+
+maintainers:
+  - Jonathan Cameron <jic23@kernel.org>
+  - Guido Günther <agx@sigxcpu.org>
+
+description: |
+  This document defines device tree properties common to several iio
+  sensors. It doesn't constitue a device tree binding specification by itself but
+  is meant to be referenced by device tree bindings.
+
+  When referenced from sensor tree bindings the properties defined in this
+  document are defined as follows. The sensor tree bindings are responsible for
+  defining whether each property is required or optional.
+
+properties:
+  proximity-near-level:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: |
+      For proximity sensors whether an object can be considered near to the
+      device depends on parameters like sensor position, covering glass and
+      aperture. This value gives an indication to userspace for which
+      sensor readings this is the case.
+
+      Raw proximity values equal or above this level should be
+      considered 'near' to the device (an object is near to the
+      sensor).
+
+...