microblaze: Fix kmalloc alignment on non-coherent DMA platforms

microblaze: Fix kmalloc alignment on non-coherent DMA platforms

Based on PowerPC patche 52142e756e9bf6485d3d53596e8aff2e816a7253

PowerPC description:
On platforms doing non-coherent DMA (4xx, 8xx, ...), it's important
that the kmalloc minimum alignment is set to the cache line size, to
avoid sharing cache lines between different objects, so that DMA to
one of the objects doesn't corrupt the other.

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