[POWERPC] Cleanup pgtable-ppc32.h

* Removed defines KERNEL_PGD_PTRS & USER_PGD_PTRS since they aren't
  used anywhere
* Changed pmd_page macro to use pfn_to_page so we get proper behavior
  if ARCH_PFN_OFFSET is set as well if we use a different memory model
  on ppc32.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>