mpc83xx: Don't define cpu_eth_init for DM eth

Don't use the legacy method of initializing the ethernet controller on
MPC83xx when DM is active.

Signed-off-by: Mario Six <mario.six@gdsys.cc>

arch/powerpc/cpu/mpc83xx/cpu.c

@@ -199,6 +199,7 @@ void watchdog_reset (void)
 }
 #endif
 
+#ifndef CONFIG_DM_ETH
 /*
  * Initializes on-chip ethernet controllers.
  * to override, implement board_eth_init()
@@ -214,6 +215,7 @@ int cpu_eth_init(bd_t *bis)
 #endif
 	return 0;
 }
+#endif /* !CONFIG_DM_ETH */
 
 /*
  * Initializes on-chip MMC controllers.