dmaengine: IOATDMA: Allocate DMA descriptor ring in contig DMA memory

Future IOATDMA hardware will take advantage of descriptors residing in
contiguous memory. Setting the descriptor ring in max config DMA memory
of 2MB. Each channel will need 2 of these chunks. This should provide 64k
of 64B descriptors.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>