phy: bcm-ns-usb3: enable MDIO in the platform specific code

When we finally start using MDIO layer then bus initialization will be
handled in a separated driver. It means our code handling this has to be
used for the platform driver only.

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