net: davinci_mdio: introduce "ti,cpsw-mdio" compat string

Introduce "ti,cpsw-mdio" compatible string for Davinci MDIO, because
it's required to distinguish the case when MDIO is part of TI CPSW to
enable features supported by TI CPSW (for example, enable PM
management).

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>