arm64: dts: renesas: r8a7795: Add DU device to DT

Add the DU device to r8a7795.dtsi in a disabled state.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>