net: dsa: mt7530: support MDB and bridge flag operations

Support port MDB and bridge flag operations.

As the hardware can manage multicast forwarding itself, offload_fwd_mark
can be unconditionally set to true.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>