net: dsa: rtl8366rb: Implement FDB accessors

This makes it possible to dump the forwarding database (FDB)
and set static forwarding rules in the RTL8366RB FDB.

The RTL8366RB has two FDB:s in hardware:

- One CAM (content addressable memory) with 8 entries that can
  hold static or autolearned rules.

- One L2 LUT (look up table) with 4 entries that can also hold
  static or autolearned rules, but also multicast rules with
  two src and dst IP numbers (multicast forwarding is currently
  not yet implemented).

In this patch we use both tables in order to provide static
forwarding.

Cc: Vladimir Oltean <olteanv@gmail.com>
Cc: Alvin Šipraga <alsi@bang-olufsen.dk>
Cc: Mauri Sandberg <sandberg@mailfence.com>
Cc: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>