examples/ipsec-secgw: add app mode worker

Add application inbound/outbound worker thread and
IPsec application processing code for event mode.

Example ipsec-secgw command in app mode:
ipsec-secgw -w 0002:02:00.0,ipsec_in_max_spi=128
-w 0002:03:00.0,ipsec_in_max_spi=128 -w 0002:0e:00.0 -w 0002:10:00.1
--log-level=8 -c 0x1 -- -P -p 0x3 -u 0x1 -f aes-gcm.cfg
--transfer-mode event --event-schedule-type parallel

Signed-off-by: Anoob Joseph <anoobj@marvell.com>
Signed-off-by: Ankur Dwivedi <adwivedi@marvell.com>
Signed-off-by: Lukasz Bartosik <lbartosik@marvell.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>