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>

arch/powerpc/lib/bootm.c

@@ -163,8 +163,8 @@ void arch_lmb_reserve(struct lmb *lmb)
 	sp = get_sp();
 	debug ("## Current stack ends at 0x%08lx\n", sp);
 
-	/* adjust sp by 1K to be safe */
-	sp -= 1024;
+	/* adjust sp by 4K to be safe */
+	sp -= 4096;
 	lmb_reserve(lmb, sp, (CONFIG_SYS_SDRAM_BASE + get_effective_memsize() - sp));
 
 	return ;