powerpc/mm: Revert "powerpc/mm: Remove DEBUG_VM_PGTABLE support on powerpc"

This reverts commit 675bceb0 ("powerpc/mm: Remove DEBUG_VM_PGTABLE support on powerpc")

All the related issues are fixed as of commit:
  f14312e1 ("mm/debug_vm_pgtable: avoid doing memory allocation with pgtable_t mapped.")

Hence re-enable it.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210318034855.74513-1-aneesh.kumar@linux.ibm.com