net: dsa: mv88e6xxx: add phylink support

Add rudimentary phylink support to mv88e6xxx.

TODO:
- needs to call phylink_mac_change() when the port link comes up/goes down.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>