dt-bindings: Clarify timing subnode use as panel-timing

Add a note in the documentation explaining when it's appropriate to use
the display-timings subnode on its own, as well as the preferred name to
use (panel-timing).

Changes in v3:
 - Added

Cc: Doug Anderson <dianders@chromium.org>
Cc: Eric Anholt <eric@anholt.net>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Jeffy Chen <jeffy.chen@rock-chips.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Stéphane Marchesin <marcheu@chromium.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: devicetree@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-rockchip@lists.infradead.org
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208174855.55620-2-seanpaul@chromium.org

Documentation/devicetree/bindings/display/panel/display-timing.txt

@@ -80,6 +80,11 @@ The parameters are defined as:
   |          |        v                            |          |       |
   +----------+-------------------------------------+----------+-------+
 
+Note: In addition to being used as subnode(s) of display-timings, the timing
+      subnode may also be used on its own. This is appropriate if only one mode
+      need be conveyed. In this case, the node should be named 'panel-timing'.
+
+
 Example:
 
 	display-timings {