microblaze: Stop stack clobbering in microblaze-generic.

A typo caused the stack and malloc regions to overlap, which prevented
mem_malloc_init() from returning. This commit makes the memory layout match
the example described in include/configs/microblaze-generic.h

Signed-off-by: Graeme Smecher <graeme.smecher@mail.mcgill.ca>
Signed-off-by: Michal Simek <monstr@monstr.eu>