net: dsa: mv88e6131: add registers access

Only 6131 was not supporting the port registers access yet. Assume such
support and use the unlock access routines in the meantime.

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