dt-bindings: rcar-dmac: Document r8a7744 support

Renesas RZ/G SoC also have the R-Car gen2/3 compatible DMA controllers.
Document RZ/G1N (also known as R8A7744) SoC bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>