net: dsa: add cross-chip bridging operations

Introduce crosschip_bridge_{join,leave} operations in the dsa_switch_ops
structure, which can be used by switches supporting interconnection.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>