sfc: bare bones TC offload on EF100

This is the absolute minimum viable TC implementation to get traffic to
 VFs and allow them to be tested; it supports no match fields besides
 ingress port, no actions besides mirred and drop, and no stats.
Example usage:
    tc filter add dev $PF parent ffff: flower skip_sw \
        action mirred egress mirror dev $VFREP
    tc filter add dev $VFREP parent ffff: flower skip_sw \
        action mirred egress redirect dev $PF
 gives a VF unfiltered access to the network out the physical port ($PF
 acts here as a physical port representor).
More matches, actions, and counters will be added in subsequent patches.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>