davinci: cleanup mdio arch code and switch to phy_id

This patch removes davinci architecture code that has now been rendered
useless by the previous patches in the MDIO separation series.

In addition, the earlier phy_mask definitions have been replaced with
corresponding phy_id definitions.

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>