dmaengine: rcar-dmac: Work around descriptor mode IOMMU errata

When descriptor memory is accessed through an IOMMU the DMADAR register
isn't initialized automatically from the first descriptor at beginning
of transfer by the DMAC like it should. Initialize it manually with the
destination address of the first chunk.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>