dt-bindings: display: renesas: Deprecate LVDS support in the DU bindings

The internal LVDS encoders now have their own DT bindings, representing
them as part of the DU is deprecated.

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>