arm64: dts: renesas: r8a77961: Add DU device nodes

This patch adds DU device nodes for R-Car M3-W+ (r8a77961) SoC.
This patch was tested on R-Car M3-W+ Salvator-XS board.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/87eendrtv1.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>