net: mvpp2: allow MDIO registration for fixed links

Currently, there are 2 valid cases for interface, PHY
and mdio relation:
  - If an interface has PHY handler, it'll call
    mdio_mii_bus_get_from_phy(), which will register
    MDIO bus.
  - If we want to use fixed-link for an interface,
    PHY handle is not defined in the DTS, and no
    MDIO is registered.

There is a third case, for some boards (with switch),
the MDIO is used for switch configuration, but the interface
itself uses fixed link. This patch allows this option by
checking if fixed-link subnode is defined, in this case,
MDIO bus is registers, but the PHY address is set to
PHY_MAX_ADDR for this interface, so this interface will
not try to access the PHY later on.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>