powerpc/mm/hugetlb: Handle hugepage size supported by hash config

W.r.t hash page table config, we support 16MB and 16GB as the hugepage
size. Update the hstate_get_psize to handle 16M and 16G.

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