powerpc/64s: Only set HAVE_ARCH_UNMAPPED_AREA when CONFIG_PPC_64S_HASH_MMU is set

When CONFIG_PPC_64S_HASH_MMU is not set, slice.c is not built and
arch_get_unmapped_area() and arch_get_unmapped_area_topdown() are
not provided because RADIX uses the generic ones.

Therefore, neither set HAVE_ARCH_UNMAPPED_AREA nor
HAVE_ARCH_UNMAPPED_AREA_TOPDOWN.

Fixes: ab57bd75 ("powerpc/mm: Move get_unmapped_area functions to slice.c")
Reported-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e438c6cc09f94085e56733ed2d6e84333c35292a.1653370913.git.christophe.leroy@csgroup.eu