drm: rcar-du: Convert live DT patches to sugar syntax

Using overlay sugar syntax makes the DTS files easier to read (and
write).

Overlay syntactic sugar for generating target-path fragments is
supported by the version of dtc supplied with the kernel since commit
50aafd60 ("scripts/dtc: Update to upstream version
v1.4.6-21-g84e414b0b5bc").

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