DT: dmaengine: renesas,rcar-dmac: document R8A77980 support

Renesas  R-Car V3H SoC has the R-Car gen3 compatible DMA controllers.
Document R-Car V3H (also known as R8A77980) SoC bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>