dt-bindings: display: renesas: dw-hdmi: Tidyup example compatible

The DT example erronously uses the "renesas,r8a7795-dw-hdmi", when the
correct value is "renesas,r8a7795-hdmi". It is furthermore missing the
generic "renesas,rcar-gen3-hdmi" compatible string. Fix it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
[Add "renesas,rcar-gen3-hdmi" and rework commit message]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>