docs/usecase: add tcp termination usecase

- Add tcp_term case setup scripts
- Add tcp_term case guide
- Support for DPDK and loopback interface

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I3603614cc6a781f735cad5993f26ea2d8659f4da