dt-bindings: renesas,rcar-dmac: Document r8a7742 support

Renesas RZ/G SoC also have the R-Car gen2/3 compatible DMA controllers.
Document RZ/G1H (also known as R8A7742) SoC bindings.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200903073813.4490-1-prabhakar.mahadev-lad.rj@bp.renesas.com


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