dm: mdio: Use ofnode_phy_is_fixed_link() API

Switch to use the ofnode_phy_is_fixed_link() API which can support
both the new and old DT bindings.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>