powerpc/mm: Make a copy of pgalloc.h for 32 and 64 book3s

This patch start to make a book3s variant for pgalloc headers. We have
multiple book3s specific changes such as:
  * 4 level page table
  * store physical address in higher level table
  * use pte_t * for pgtable_t

Having a book3s64 specific variant helps to keep code simpler and remove
lots of #ifdef around code.

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