dmaengine: mv_xor_v2: set DMA mask to 40 bits

The XORv2 engine on Armada 7K/8K can only access the first 40 bits of
the physical address space, so the DMA mask must be set accordingly.

Fixes: 19a340b1 ("dmaengine: mv_xor_v2: new driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>