dt-bindings: dmaengine: rcar-dmac: Document R8A774B1 bindings

Renesas RZ/G2N (R8A774B1) SoC also has the R-Car gen2/3 compatible
DMA controllers, therefore document RZ/G2N specific bindings.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1569580629-55677-1-git-send-email-biju.das@bp.renesas.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

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

@@ -21,6 +21,7 @@ Required Properties:
 		- "renesas,dmac-r8a7745" (RZ/G1E)
 		- "renesas,dmac-r8a77470" (RZ/G1C)
 		- "renesas,dmac-r8a774a1" (RZ/G2M)
+		- "renesas,dmac-r8a774b1" (RZ/G2N)
 		- "renesas,dmac-r8a774c0" (RZ/G2E)
 		- "renesas,dmac-r8a7790" (R-Car H2)
 		- "renesas,dmac-r8a7791" (R-Car M2-W)