net: phy: make mdio_device.c part of libphy

net: phy: make mdio_device.c part of libphy

This patch
- makes mdio_device.c part of libphy
- makes mdio_device_(un)register_reset() static
- moves mdiobus_(un)register_device() from mdio_bus.c to mdio_device.c,
  stops exporting both functions and makes them private to phylib

This further decouples the MDIO consumer functionality from libphy.

Note: This makes MDIO driver registration part of phylib, therefore
      adjust Kconfig dependencies where needed.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/c6dbf9b3-3ca0-434b-ad3a-71fe602ab809@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>