arm64: dts: renesas: r8a77995: add SYS-DMAC nodes

Differs from other Gen3 SoCs in that each controller only supports eight
channels.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>