arm64: dts: renesas: r8a779a0: Add SYS-DMAC nodes

Add device nodes for the Direct Memory Access Controller for System
(SYS-DMAC) instances on the Renesas R-Car V3U (r8a779a0) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210107182045.1948037-1-geert+renesas@glider.be