Documentation: networking: cpsw: add MQPRIO & CBS offload examples

This document describes MQPRIO and CBS Qdisc offload configuration
for cpsw driver based on examples. It potentially can be used in
audio video bridging (AVB) and time sensitive networking (TSN).
Reviewed-by: NIlias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: NGrygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: NIvan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>