drivers: net: davinci_mdio: enable pm runtime auto for ti cpsw-mdio

Use "ti,cpsw-mdio" to enable PM runtime auto-suspend on supported
platforms, where MDIO is implemented as part of TI CPSW.

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