net: davinci_emac: separate out davinci mdio

Davinci's MDIO controller is present on other TI devices, without an
accompanying EMAC.  For example, on tnetv107x, the same MDIO module is used in
conjunction with a 3-port switch hardware.

By separating the MDIO controller code into its own platform driver, this
patch allows common logic to be reused on such platforms.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Tested-by: Michael Williamson <michael.williamson@criticallink.com>
Tested-by: Caglar Akyuz <caglarakyuz@gmail.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>