net: dsa: rtl8366rb: Support fast aging

This implements fast aging per-port using the special "security"
register, which will flush any learned L2 LUT entries on a port.

The vendor API just enabled setting and clearing this bit, so
we set it to age out any entries on the port and then we clear
it again.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Cc: Mauri Sandberg <sandberg@mailfence.com>
Cc: DENG Qingfang <dqfext@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>