dt-bindings: dmaengine: rcar-dmac: Document r8a77961 support

Document support for the system DMA controller in the Renesas R-Car
M3-W+ (R8A77961) SoC.

No driver update is needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191205133736.5934-1-geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>