net: dsa: mv88e6xxx: Refactor MDIO so driver registers mdio bus

Have the switch driver register its own MDIO bus. This allows for an
mdio property in the device tree, with child nodes for phys, which
can be referenced via phandles, etc.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>