dt-bindings: dma: renesas,rcar-dmac: Document R8A774E1 bindings

Renesas RZ/G2H (R8A774E1) SoC also has the R-Car gen3 compatible
DMA controllers, therefore document RZ/G2H specific bindings.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1594676120-5862-5-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com


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