rocker: adding port ageing_time for ageing out FDB entries

Follow-up patcheset will allow user to change ageing_time, but for now
just hard-code it to a fixed value (the same value used as the default
for the bridge driver).

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>