net: dsa: b53: Add bridge support

Add support for HW bridging by tying the ports together in the same port
VLAN mask when they belong to the same bridge, and isolating them to be
alone with the CPU port when they are not.

Propagate STP states from the bridge layer to the switch's HW mapping
when requested.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>