phy: bcm-ns-usb3: add MDIO driver using proper bus layer

As USB 3.0 PHY is attached to the MDIO bus this module should provide a
MDIO driver and use a proper bus layer. This is a proper (cleaner)
solution which doesn't require code to know this specific MDIO bus
details. It also allows reusing the driver with other MDIO buses.

For now keep platform device support in place. We may consider dropping
it once MDIO bindings gets used "everywhere".

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>