dmaengine: mv_xor_v2: implement proper interrupt coalescing

Until now, the driver was not using interrupt coalescing: one interrupt
was generated for each descriptor processed by the XOR engine. This
commit changes that by using the interrupt coalescing features of the
hardware, by setting both a number of descriptors processed before an
interrupt is generated and a timeout before an interrupt is generated.

Signed-off-by: Hanna Hawa <hannah@marvell.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>