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

Documentation/features/debug/debug-vm-pgtable/arch-support.txt

@@ -21,7 +21,7 @@
     |       nios2: | TODO |
     |    openrisc: | TODO |
     |      parisc: | TODO |
-    |     powerpc: | TODO |
+    |     powerpc: |  ok  |
     |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: | TODO |

arch/powerpc/Kconfig

@@ -119,6 +119,7 @@ config PPC
 	#
 	select ARCH_32BIT_OFF_T if PPC32
 	select ARCH_HAS_DEBUG_VIRTUAL
+	select ARCH_HAS_DEBUG_VM_PGTABLE
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FORTIFY_SOURCE