arm64: dts: renesas: rzg2: Add reset control properties for display

Add reset control properties to the device nodes for the Display Units
on all supported RZ/G2 SoCs.  Note that on these SoCs, there is only a
single reset for each pair of DU channels.

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

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20200218133019.22299-5-geert+renesas@glider.be