ppc: unused memory region too close to current stack pointer

This avoids a possible overwrite of the (end of) ramdisk by u-boot.
The unused memory region for ppc boot currently starts 1k below the
do_bootm->bootm_start->arch_lmb_reserve stack ptr. This isn't enough since
do_bootm->do_bootm_linux->boot_relocate_fdt calls printf which may
very well use more than 1k stack space.

Signed-off-by: Norbert van Bolhuis <nvbolhuis@aimvalley.nl>