net: dsa: mv88e6xxx: convert 88e639x to phylink_pcs

Convert the 88E6390, 88E6390X, and 88E6393X family of switches to use
the phylink_pcs infrastructure.

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