net: dsa: mv88e6xxx: Support multiple MDIO busses

The mv88e6390 has multiple MDIO busses. Generalize the parsing of the
device tree to support multiple mdio nodes. The external mdio bus has
a compatible strings to indicate it is external.

Keep a linked list of busses, placing the external mdio bus at the
tail of the list. When within the driver an mdio bus is needed,
e.g. for EEE or SERDES, use the head of the list which should be the
internal bus.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>