mpc83xx: Set guarded bit on BAT that covers the end of the address space

The mpc8313erdb board currently sets DBAT6 to cover all of the final 256MiB of
address space; however, not all of this space is covered by a device.  In
particular, flash sits at 0xfe000000-0xfe7fffff, and nothing is mapped
at the far end of the address space.

In zlib, there is a loop that references p[-1] if p is non-NULL.  Under
some circumstances, this leads to the CPU speculatively loading from
0xfffffff8 if p is NULL.  This leads to a machine check.

Signed-off-by: Scott Wood <scottwood@freescale.com>

continuation to the remaining mpc83xx boards that suffer from the same problem.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>