net: dsa: sja1105: make room for virtual link parsing in flower offload

Virtual links are a sja1105 hardware concept of executing various flow
actions based on a key extracted from the frame's DMAC, VID and PCP.

Currently the tc-flower offload code supports only parsing the DMAC if
that is the broadcast MAC address, and the VLAN PCP. Extract the key
parsing logic from the L2 policers functionality and move it into its
own function, after adding extra logic for matching on any DMAC and VID.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>