net: dsa: mv88e6xxx: Support policy entries in the VTU

A VTU entry with policy enabled is used in combination with a port's
VTU policy setting to override normal switching behavior for frames
assigned to the entry's VID.

A typical example is to Treat all frames in a particular VLAN as
control traffic, and trap them to the CPU. In which case the relevant
user port's VTU policy would be set to TRAP.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>