ARM: dts: keystone: add mdio devices entries

The Keystone 2 has MDIO HW block which are compatible
to Davinci SoCs:
See "Gigabit Ethernet (GbE) Switch Subsystem"
  See http://www.ti.com/lit/ug/sprugv9d/sprugv9d.pdf



Hence, add corresponding DT entry for Keystone 2.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>