dt-bindings: drm: Introduce the DP AUX bus

We want to be able to list an eDP panel as a child of an eDP
controller node to represent the fact that the panel is connected to
the controller's DP AUX bus. Though the panel and the controller are
connected in several ways, the DP AUX bus is the primary control
interface between the two and thus makes the most sense to model in
device tree hierarchy.

Listing a panel in this way makes it possible for the panel driver to
easily get access to the DP AUX bus that it resides on, which can be
useful to help in auto-detecting the panel and for turning on various
bits.

NOTE: historically eDP panels were _not_ listed under their controller
but were listed at the top level of the device tree. This will still
be supported for backward compatibility (and while DP controller
drivers are adapted to support the new DT syntax) but should be
considered deprecated since there is no downside to listing the panel
under the controller.

For now, the DP AUX bus bindings will only support an eDP panel
underneath. It's possible it could be extended to allow having a DP
connector under it in the future.

NOTE: there is no "Example" in this bindings file. Yikes! This avoids
duplicating the same example lots of places. See users of the aux bus
(like ti-sn65dsi86) for examples.

The idea for this bus's design was hashed out over IRC [1].

[1] https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&date=2021-05-11Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611101711.v10.2.Id3c048d22e72a9f90084a543b5b4e3f43bc9ab62@changeid

Documentation/devicetree/bindings/display/dp-aux-bus.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/dp-aux-bus.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort AUX bus
+
+maintainers:
+  - Douglas Anderson <dianders@chromium.org>
+
+description:
+  DisplayPort controllers provide a control channel to the sinks that
+  are hooked up to them. This is the DP AUX bus. Over the DP AUX bus
+  we can query properties about a sink and also configure it. In
+  particular, DP sinks support DDC over DP AUX which allows tunneling
+  a standard I2C DDC connection over the AUX channel.
+
+  To model this relationship, DP sinks should be placed as children
+  of the DP controller under the "aux-bus" node.
+
+  At the moment, this binding only handles the eDP case. It is
+  possible it will be extended in the future to handle the DP case.
+  For DP, presumably a connector would be listed under the DP AUX
+  bus instead of a panel.
+
+properties:
+  $nodename:
+    const: "aux-bus"
+
+  panel:
+    $ref: panel/panel-common.yaml#
+
+additionalProperties: false
+
+required:
+  - panel