enetc: Clean up MAC and link configuration

Decouple level MAC configuration based on phy interface type
from general port configuration.
Group together MAC and link configuration code.
Decouple external mdio bus creation from interface type
parsing.  No longer return an (unhandled) error code when
phy_node not found, use phy_node to indicate whether the
port has a phy or not.  No longer fall-through when serdes
configuration fails for the link modes that require
internal link configuration.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>