dt-bindings: Add headings to simple-panel bindings

In preparation for a new subnode section in a follow-on patch, add
explicit headings to the existings sections for simple-panel.

Changes in v2:
 - Added
Changes in v3:
 - None

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>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208174855.55620-3-seanpaul@chromium.org

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

 Simple display panel
+====================
+
+panel node
+----------
 
 Required properties:
 - power-supply: See panel-common.txt