dmaengine: Provide a wrapper for memcpy operations

Almost all ->device_prep_dma_xx() methods have a wrapper defined in
dmaengine.h. Add one for  ->device_prep_dma_memcpy().

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>