net: dsa: mv88e6xxx: add infrastructure for phylink_pcs

Add infrastructure for phylink_pcs to the mv88e6xxx driver. This
involves adding a mac_select_pcs() hook so we can pass the PCS to
phylink at the appropriate time, and a PCS initialisation function.

As the various chip implementations are converted to use phylink_pcs,
they are no longer reliant on the legacy phylink behaviour. We detect
this by the use of this infrastructure, or the lack of any serdes.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>