net: dsa: integrate with SWITCHDEV for HW bridging

In order to support bridging offloads in DSA switch drivers, select
NET_SWITCHDEV to get access to the port_stp_update and parent_get_id
NDOs that we are required to implement.

To facilitate the integratation at the DSA driver level, we implement 3
types of operations:

- port_join_bridge
- port_leave_bridge
- port_stp_update

DSA will resolve which switch ports that are currently bridge port
members as some Switch hardware/drivers need to know about that to limit
the register programming to just the relevant registers (especially for
slow MDIO buses).

We also take care of setting the correct STP state when slave network
devices are brought up/down while being bridge members.

Finally, when a port is leaving the bridge, we make sure we set in
BR_STATE_FORWARDING state, otherwise the bridge layer would leave it
disabled as a result of having left the bridge.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>