dmaengine: rcar-dmac: Add device tree bindings documentation

Document the device tree bindings for the R-Car Generation 2 DMA
Controller (DMAC).

Cc: devicetree@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>