net: dsa: mv88e6xxx: extend phylink to Serdes PHYs

Extend the mv88e6xxx phylink implementation down to Serdes PHYs, which
handle the PCS layer of such links.

- Implement phylink PCS link state reading, so that we can provide
  ethtool with the linkmodes and link speed in the expected manner.
  Note: this will only be called for in-band negotiation, which is
  only supported by the serdes interfaces.
- Implement phylink PCS configuration, so that the in-band AN and
  advertisement can be configured.
- Implement phylink PCS negotiation restart, so that the in-band AN
  can be restarted.
- Implement phylink PCS link up, so that when operating out-of-band,
  the Serdes can be configured for the appropriate fixed speed mode.

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