net: am65-cpsw: cpsw_mdio: Switch to proper DM_MDIO framework

Add a new Kconfig symbol MDIO_TI_CPSW for the CPSW MDIO
driver and build it with proper DM support if enabled.

If MDIO_TI_CPSW is not enabled then we continue to
behave like before.

Clean up MDIO custom handling in am65-cpsw and use
dm_eth_phy_connect() to get the PHY.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Tested-by: Ravi Gunasekaran <r-gunasekaran@ti.com>