powerpc/mm/radix: Add hugetlb support 4K page size

We have hugepage at the pmd level with 4K radix config. Hence we don't
need to use hugepd format with radix.

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