microblaze: Define ARCH_SLAB_MINALIGN to fix slab crash

The commit "mm: Move ARCH_SLAB_MINALIGN and
ARCH_KMALLOC_MINALIGN to <linux/slab_def.h>"
1f0ce8b3 which moved the ARCH_SLAB_MINALIGN
default into the global header broke FLAT for Microblaze.

Error message:
slab error in verify_redzone_free(): cache `idr_layer_cache':
memory outside object was overwritten

Signed-off-by: Michal Simek <monstr@monstr.eu>