ARM: dts: am335x/am437x/dra7: use new "ti, cpsw-mdio" compat string

Add "ti,cpsw-mdio" for am335x/am437x/dra7 SoCs where MDIO is
implemented as part of TI CPSW and, this way, enable PM runtime auto
suspend for Davinci MDIO driver on these paltforms.

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