arm64: dts: renesas: r8a77961: Add SYS-DMAC nodes

Add device nodes for the System DMA Controllers (SYS-DMAC) on the
Renesas R-Car M3-W+ (r8a77961) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20191216124740.16647-5-geert+renesas@glider.be