net: dsa: Associate slave network device with CPU port

In preparation for supporting multiple CPU ports with DSA, have the
dsa_port structure know which CPU it is associated with. This will be
important in order to make sure the correct CPU is used for transmission
of the frames. If not for functional reasons, for performance (e.g: load
balancing) and forwarding decisions.

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>