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>

arch/powerpc/cpu/mpc8xxx/cpu.c

@@ -343,6 +343,7 @@ int fixup_cpu(void)
 	return 0;
 }
 
+#ifndef CONFIG_DM_ETH
 /*
  * Initializes on-chip ethernet controllers.
  * to override, implement board_eth_init()
@@ -370,3 +371,4 @@ int cpu_eth_init(struct bd_info *bis)
 #endif
 	return 0;
 }
+#endif