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: NGeert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191205133736.5934-1-geert+renesas@glider.beSigned-off-by: NVinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/renesas,rcar-dmac.txt

@@ -30,6 +30,7 @@ Required Properties:
 		- "renesas,dmac-r8a7794" (R-Car E2)
 		- "renesas,dmac-r8a7795" (R-Car H3)
 		- "renesas,dmac-r8a7796" (R-Car M3-W)
+		- "renesas,dmac-r8a77961" (R-Car M3-W+)
 		- "renesas,dmac-r8a77965" (R-Car M3-N)
 		- "renesas,dmac-r8a77970" (R-Car V3M)
 		- "renesas,dmac-r8a77980" (R-Car V3H)