powerpc/86xx: Increase boot map size to 256 MiB

This is what Linux maps on classic PPC during boot, and modern kernel
images don't fit within the current 8 MiB uncompressed limit.

Adjust image load addresses to be above this limit to avoid conflicts.

Signed-off-by: Scott Wood <oss@buserror.net>
Reviewed-by: York Sun <york.sun@nxp.com>