net: dsa: felix: add support Credit Based Shaper(CBS) for hardware offload

VSC9959 hardware support the Credit Based Shaper(CBS) which part
of the IEEE-802.1Qav. This patch support sch_cbs set for VSC9959.

Signed-off-by: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>