net: phy: Allow building mdio-boardinfo into the kernel

mdio-boardinfo contains code that is helpful for platforms to register
specific MDIO bus devices independent of how CONFIG_MDIO_DEVICE or
CONFIG_PHYLIB will be selected (modular or built-in). In order to make
that possible, let's do the following:

- descend into drivers/net/phy/ unconditionally

- make mdiobus_setup_mdiodev_from_board_info() take a callback argument
  which allows us not to expose the internal MDIO board info list and
  mutex, yet maintain the logic within the same file

- relocate the code that creates a MDIO device into
  drivers/net/phy/mdio_bus.c

- build mdio-boardinfo.o into the kernel as soon as MDIO_DEVICE is
  defined (y or m)

Fixes: 90eff909 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Fixes: 648ea013 ("net: phy: Allow pre-declaration of MDIO devices")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>