net: dsa: microchip: add ETS Qdisc support for KSZ9477 series

Add ETS Qdisc support for KSZ9477 of switches. Current implementation is
limited to strict priority mode.

Tested on KSZ8563R with following configuration:
tc qdisc replace dev lan2 root handle 1: ets strict 4 \
  priomap 3 3 2 2 1 1 0 0
ip link add link lan2 name v1 type vlan id 1 \
  egress-qos-map 0:0 1:1 2:2 3:3 4:4 5:5 6:6 7:7

and patched iperf3 version:
https://github.com/esnet/iperf/pull/1476


iperf3 -c 172.17.0.1 -b100M  -l1472 -t100 -u -R --sock-prio 2

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>