dt-bindings: display/panel: Add common rotation property

Display panels can be oriented many ways, especially in the embedded
world. The rotation property is a way to describe this orientation.
The counter clockwise direction is chosen because that's what fbdev
and drm use.
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>

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

+Common display properties
+-------------------------
+
+- rotation:	Display rotation in degrees counter clockwise (0,90,180,270)