enetc: add support Credit Based Shaper(CBS) for hardware offload

The ENETC hardware support the Credit Based Shaper(CBS) which part
of the IEEE-802.1Qav. The CBS driver was loaded by the sch_cbs
interface when set in the QOS in the kernel.

Here is an example command to set 20Mbits bandwidth in 1Gbits port
for taffic class 7:

tc qdisc add dev eth0 root handle 1: mqprio \
	   num_tc 8 map 0 1 2 3 4 5 6 7 hw 1

tc qdisc replace dev eth0 parent 1:8 cbs \
	   locredit -1470 hicredit 30 \
	   sendslope -980000 idleslope 20000 offload 1

Signed-off-by: Po Liu <Po.Liu@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>