net: dsa: bcm_sf2: Split fast age into a helper function

Add a helper function to fast age something that is controlled by the
caller: port, VLAN. We will use this to implement a VLAN fast age
operation.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>