dmaengine: mv_xor: Enlarge descriptor pool size

Now that we have 2 channels assigned to 2 CPUs and all requests are chained
on same channels, we need much more descriptors available to satisfy
async_tx workload.

3072 descriptors was found in our lab as the number of descriptors which
allow the async_tx stack to work without waiting for free descriptors on
submission of new requests.

Signed-off-by: Lior Amsalem <alior@marvell.com>
Reviewed-by: Nadav Haklai <nadavh@marvell.com>
Tested-by: Nadav Haklai <nadavh@marvell.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>