powerpc/mm/hash/4k: Don't use 64K page size for vmemmap with 4K pagesize

With hash translation and 4K PAGE_SIZE config, we need to make sure we don't
use 64K page size for vmemmap.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>