mpc8xxx: Don't compile board_eth_init() for DM_ETH

The cpu_eth_init() is only used by the legacy ethernet driver framework.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>