powerpc: Fix build error with 64K pages and !hugetlbfs

HAVE_ARCH_UNMAPPED_AREA and HAVE_ARCH_UNMAPPED_AREA_TOPDOWN must
be defined whenever CONFIG_PPC_MM_SLICES is enabled, not just when
CONFIG_HUGETLB_PAGE is.  They used to be always defined together but
this is no longer the case since 3a8247cc
("powerpc: Only demote individual slices rather than whole process").

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>