powerpc: Only define HAVE_ARCH_HUGETLB_UNMAPPED_AREA if PPC_MM_SLICES

If we don't have slices, we should be able to use the generic
hugetlb_get_unmapped_area() code

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>