Documentation: networking: add cpsw switchdev based driver documentation

A new cpsw dirver based on switchdev was added. Add documentation about
basic configuration and future features

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>