net: davinci_mdio: allow to create phys from dt

This patch allows to create PHYs from DT in case
if they are explicitly defined. The of_mdiobus_register() is
used for such purposes.

For backward compatibility, call  of_mdiobus_register() only in case
if at least one PHY's child is defined in DT, otherwise rollback to
mdiobus_register().

Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>