powerpc/mm: Revert changes made to nohash pgalloc-64.h

This reverts pgalloc related changes WRT implementing 4-level page
table for 64K Linux page size and storing of physical address in higher
level page tables since they are only applicable to book3s64 variant
and we now have a separate copy for book3s64. This helps to keep these
headers simpler.

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