examples/ipsec-secgw: add framework for event helper

examples/ipsec-secgw: add framework for event helper

Add framework for eventmode helper. Event mode involves
initialization of multiple devices like eventdev, ethdev
and etc. Add routines to initialize and uninitialize event
device. Generate a default config for event device if it
is not specified in the configuration. Currently event
helper supports single event device only.

Signed-off-by: Anoob Joseph <anoobj@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>