dma: mv_xor: Simplify the DMA_MEMCPY operation

A memory copy operation can be expressed as an XOR operation with one
source. This commit removes code duplication in the driver by reusing
the XOR operation for the MEMCPY.

As an added benefit, we can now put MEMCPY and XOR descriptors on the
same chain, which improves performance.

Signed-off-by: Lior Amsalem <alior@marvell.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>