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

The M3-W HDMI TX controller seems to be compatible for the 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: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>