net: mdio-uclass: Bind and probe generic Ethernet PHY driver

If DM_ETH_PHY is enabled then try to bind and probe the
generic Ethernet PHY driver for each child of MDIO bus.

This is to ensure that GPIO reset handling is done if available
before MDIO bus driver scans for the PHYs.

Signed-off-by: Roger Quadros <rogerq@kernel.org>