intel_mid_dma: add support for single item scatter-gather list

Current driver's device_prep_slave_sg can't be used by DMAC2 even
the sg list contains one item, this patch will enable DMAC2 to
use this API.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>