mpc512x: make MEM IO Control configuration a board config option

Signed-off-by: Anatolij Gustschin <agust@denx.de>
diff --git a/include/configs/mecp5123.h b/include/configs/mecp5123.h
index cccc31d..cafd6a7 100644
--- a/include/configs/mecp5123.h
+++ b/include/configs/mecp5123.h
@@ -67,6 +67,8 @@
 #define CONFIG_SYS_DDR_BASE		0x00000000	/* DDR is sys memory*/
 #define CONFIG_SYS_SDRAM_BASE		CONFIG_SYS_DDR_BASE
 
+#define CONFIG_SYS_IOCTRL_MUX_DDR	0x00000036
+
 /* DDR Controller Configuration
  *
  * SYS_CFG: