powerpc/mm: Move page table index and and vaddr to pgtable.h

Now that the page table size is a variable, we can move these to
generic pgtable.h.

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