dt-bindings: display: renesas: Add R-Car M3-N HDMI TX DT bindings

The M3-N HDMI TX controller is compatible with the M3-W and H3. No
extension to the DT bindings are needed.

Add an SoC-specific compatible string in case differences between the IP
versions are found later and require model-specific handling.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>