ARM: dts: omap3: name mdio node properly

Rename davinci_mdio DT node "ethernet"->"mdio"
This fixes the following DT schemas check errors:
am3517-craneboard.dt.yaml: ethernet@5c030000: $nodename:0: 'ethernet@5c030000' does not match '^mdio(@.*)?'

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>