dmaengine: rcar-dmac: Document R-Car D3 bindings

R8A77995's SYS-DMAC is R-Car Gen3-compatible.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
[simon: rebased]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>