net: ethernet: ti: am65-cpsw-nuss: use of_platform_device_create() for mdio

The MCU CPSW expected to populate only MDIO device, but follow up patches
will add "compatible" property to the MCU CPSW CPTS node which will cause
creation of CPTS device and MCU CPSW init failure. Hence, switch to use
of_platform_device_create() instead of of_platform_populate() for MDIO
device population.

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