dmaengine: rcar-dmac: Document R8A774A1 bindings

Renesas' RZ/G2M (R8A774A1) SoC has DMA controllers compatible
with this driver, therefore document RZ/G2M specific bindings.
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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

 * Renesas R-Car (RZ/G) DMA Controller Device Tree bindings
 
-Renesas R-Car Generation 2 SoCs have multiple multi-channel DMA
+Renesas R-Car (Gen 2/3) and RZ/G SoCs have multiple multi-channel DMA
 controller instances named DMAC capable of serving multiple clients. Channels
 can be dedicated to specific clients or shared between a large number of
 clients.
@@ -20,6 +20,7 @@ Required Properties:
 		- "renesas,dmac-r8a7744" (RZ/G1N)
 		- "renesas,dmac-r8a7745" (RZ/G1E)
 		- "renesas,dmac-r8a77470" (RZ/G1C)
+		- "renesas,dmac-r8a774a1" (RZ/G2M)
 		- "renesas,dmac-r8a7790" (R-Car H2)
 		- "renesas,dmac-r8a7791" (R-Car M2-W)
 		- "renesas,dmac-r8a7792" (R-Car V2H)