examples/ipsec-secgw: add cryptodev mask option

examples/ipsec-secgw: add cryptodev mask option

Previously, ipsec-secgw application did not give user the
flexibility to decide which crypto device(s) will be used.

In this patch, a new cryptodev_mask option is added to the
application. Same as portmask, the cryptodev_mask avails the
user to mask out the unwanted crypto devices in the system.

This patch is similar to the support added in l2fwd-crypto
(d2797f51cc63: examples/l2fwd-crypto: add cryptodev mask option)

Signed-off-by: Akhil Goyal <akhil.goyal@nxp.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>