dt-bindings: display: renesas: Add R-Car LVDS encoder DT bindings

The Renesas R-Car Gen2 and Gen3 SoCs have internal LVDS encoders. Add
corresponding device tree bindings.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>