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: 90eff9096c01 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Fixes: 648ea0134069 ("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>
diff --git a/drivers/net/Makefile b/drivers/net/Makefile
index 55f75ae..57fc47a 100644
--- a/drivers/net/Makefile
+++ b/drivers/net/Makefile
@@ -18,7 +18,7 @@
 obj-$(CONFIG_MDIO) += mdio.o
 obj-$(CONFIG_NET) += Space.o loopback.o
 obj-$(CONFIG_NETCONSOLE) += netconsole.o
-obj-$(CONFIG_MDIO_DEVICE) += phy/
+obj-y += phy/
 obj-$(CONFIG_RIONET) += rionet.o
 obj-$(CONFIG_NET_TEAM) += team/
 obj-$(CONFIG_TUN) += tun.o