examples/ipsec-secgw: support event Tx adapter

Add Tx adapter support. The event helper init routine will initialize
the Tx adapter according to the configuration. If Tx adapter config
is not present it will generate a default config.

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>