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>