net: phy: make mdio consumer / device layer a separate module

After having factored out the provider part from mdio_bus.c, we can
make the mdio consumer / device layer a separate module. This also
allows to remove Kconfig symbol MDIO_DEVICE.
The module init / exit functions from mdio_bus.c no longer have to be
called from phy_device.c. The link order defined in
drivers/net/phy/Makefile ensures that init / exit functions are called
in the right order.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/dba6b156-5748-44ce-b5e2-e8dc2fcee5a7@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>