net: dsa: b53: Add PHYLINK support

Add support for PHYLINK, things are reasonably straight forward since we
do not yet support SerDes interfaces, that leaves us with just
MLO_AN_PHY and MLO_AN_FIXED to deal with.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>