dt-bindings: renesas,rcar-dmac: Add r8a779a0 support

Document the compatible value for the Direct Memory Access Controller
blocks in the Renesas R-Car V3U (R8A779A0) SoC.

The most visible difference with DMAC blocks on other R-Car SoCs is the
move of the per-channel registers to a separate register block.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20210128084455.2237256-2-geert+renesas@glider.be


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