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>