net: stmmac: add RX frame steering based on VLAN priority in tc flower

We extend tc flower to support configuration of VLAN priority-based RX
frame steering hardware offloading.

To map VLAN <PCP> to Traffic Class <TC>:
  $ tc filter add dev <IFNAME> parent ffff: protocol 802.1Q flower \
       vlan_prio <PCP> hw_tc <TC>

  Note: <TC> < N whereby "tc qdisc ... num_tc N ..."

To delete all tc flower configurations:
  $ tc qdisc delete dev <IFNAME> ingress

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>