net: dsa: mv88e6xxx: add PHYLINK support

Add rudimentary phylink support to mv88e6xxx. This allows the driver
using user ports with fixed links to keep operating normally. User ports
with normal PHYs are not affected since the switch automatically manages
their link parameters. User facing ports which use a SFP/SFF with a
non-fixed link mode might require a call to phylink_mac_change() to
operate properly.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
[Andrew: fixed link setting after adding link polling]
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
[florian: expand commit message]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>