dmaengine: rcar-dmac: enable descriptor mode on 40bit

SYS-DMAC can use 40bit address transfer, and it supports Descriptor
Mode too. Current SYS-DMAC driver disables Descriptor Mode if it was
40bit address today. But it can use Descriptor Mode with 40bit if
transfer Source/Destination address are located in same 4GiB region
in the 40 bit address space.
This patch enables it if all condition was clear

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>