arm64: dts: renesas: r8a77995: Add cpg reset for DU

Add CPG reset properties to DU node of D3 (r8a77995) SoC.

According to Laurent Pinchart, R-Car Gen3 reset is handled at the group
level so specifying one reset entry per group is sufficient.

This patch was inspired by a patch in the BSP by
Takeshi Kihara <takeshi.kihara.df@renesas.com>.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>