dt-bindings: display: renesas: Add R-Car Gen3 HDMI TX DT bindings

The Renesas R-Car Gen3 SoCs use a Synopsys DWC HDMI TX encoder IP. Add
corresponding device tree bindings based on the DWC HDMI TX bindings
model.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>