[POWERPC] bootwrapper: cuboot-83xx: Exclude upper 1MB from heap.

The uppermost part of memory is where u-boot puts the stack, so don't
include that in the heap.  It's not currently causing problems, as the
current code allocates from the bottom of the heap, but this will keep
things from potentially breaking if a future implementation were to
allocate from the top.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>