Normally ethtool is used to acces MDIO registers of the PHY which though reguires a MAC interface to access the corresponding PHY. Reading your post that sounds like what you are trying to do. Looking through the MDIO driver it expects to couple with an Ethernet MAC driver. I don't see a way to do what you are tring to do other than writing a MAC driver that interfaces with the Davinci MDIO driver.
Were you expecting to use ethtool?