powerpc: Reduce the PTE_INDEX_SIZE

This make one PMD cover 16MB range. That helps in easier implementation of THP
on power. THP core code make use of one pmd entry to track the hugepage and
the range mapped by a single pmd entry should be equal to the hugepage size
supported by the hardware.

This also switch PGD to cover 16GB. That is needed so that we can simplify the
hugetlb page walking code so that we have same pte format for explicit hugepage
and THP hugepage.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>