examples/ipsec-secgw: add option for descriptors per QP

examples/ipsec-secgw: add option for descriptors per QP

Added option to configure number of queue pair descriptors via command
line (--desc-nb NUMBER_OF_DESC).

When the crypto processing takes longer durations, small queue pair size
would result in cryptodev enqueue failures. Larger queue pair size would
allow more packets to stay in flight simultaneously and reduce enqueue
failures.

Signed-off-by: Volodymyr Fialko <vfialko@marvell.com>
Acked-by: Anoob Joseph <anoobj@marvell.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>