net: dsa: rtl8366rb: Support bridge offloading

Use port isolation registers to configure bridge offloading.

Tested on the D-Link DIR-685, switching between ports and
sniffing ports to make sure no packets leak.

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