dmaengine: rcar-dmac: document R8A77970 bindings

Renesas R-Car V3M (R8A77970) SoC also has the R-Car gen2/3 compatible DMA
controllers, so document the SoC specific binding.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>