net: dsa: mv88e6xxx: helper to setup upstream port

Add a helper function to setup the upstream port of a given port.

This is the port used to reach the dedicated CPU port. This function
will be extended later to setup the global upstream port as well.

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