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