powerpc/book3s_32: Fix build error with checkpoint restart

In file included from mm/vmscan.c:54:0:
include/linux/swapops.h: In function ‘pte_to_swp_entry’:
include/linux/swapops.h:69:2: error: implicit declaration of function ‘pte_swp_soft_dirty’ [-Werror=implicit-function-declaration]
  if (pte_swp_soft_dirty(pte))
  ^
include/linux/swapops.h:70:3: error: implicit declaration of function ‘pte_swp_clear_soft_dirty’ [-Werror=implicit-function-declaration]
   pte = pte_swp_clear_soft_dirty(pte);

We support soft dirty tracking only with book3s 64 for now.
So change the Kconfig dependency accordingly. Also CHECKPOINT_RESTORE
feature is not really dependent on SOFT_DIRTY. We track the dependency
between MEM_SOFT_DIRTY and ARCH_SOFT_DIRTY through headers

Fixes: 7207f436 ("powerpc/mm: Add page soft dirty tracking")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>