mpc83xx: fix the corruption of u-boot when saveenv

Robert P. J. Day has pointed that the value of SYS_MONITOR_LEN in
MPC8315ERDB.h is smaller than the u-boot.bin. This will cause the
overlap between the code of u-boot and the environment variable.
So when executing saveenv, it will corrupt the code of u-boot and
causes the board not boot. Fix this for all the mpc83xx boards by
reserving a 512K area.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/include/configs/MPC8315ERDB.h b/include/configs/MPC8315ERDB.h
index 7ce5f59..5b23f94 100644
--- a/include/configs/MPC8315ERDB.h
+++ b/include/configs/MPC8315ERDB.h
@@ -169,7 +169,7 @@
 /*
  * The reserved memory
  */
-#define CONFIG_SYS_MONITOR_LEN	(384 * 1024) /* Reserve 384 kB for Mon */
+#define CONFIG_SYS_MONITOR_LEN	(512 * 1024) /* Reserve 512 kB for Mon */
 #define CONFIG_SYS_MALLOC_LEN	(512 * 1024) /* Reserved for malloc */
 
 /*