ARM: dts: rcar-gen2: Add reset control properties for display

Add reset control properties to the device nodes for the Display Units
on all supported R-Car Gen2 SoCs.  Note that on these SoCs, there is
only a single reset for all DU channels.

Join the clocks lines while at it, to increase uniformity.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20200218133019.22299-2-geert+renesas@glider.be