powerpc/mm/hugetlb: Allow runtime allocation of 16G.

Now that we have GIGANTIC_PAGE enabled on powerpc, use this for 16G hugepages
with hash translation mode. Depending on the total system memory we have, we may
be able to allocate 16G hugepages runtime. This also remove the hugetlb setup
difference between hash/radix translation mode.

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