powerpc/mm: Fix HAVE_ARCH_SOFT_DIRTY dependencies

[Ben Hutchings]

Soft dirty bit support was only implemented for 64-bit Book3S, and
32-bit configurations currently fail to build.

Fixes: 7207f43665b8 ("powerpc/mm: Add page soft dirty tracking")
References: https://buildd.debian.org/status/fetch.php?pkg=linux&arch=powerpc&ver=4.5%7Erc4-1%7Eexp1&stamp=1455791718
Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
---
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -557,7 +557,7 @@ choice
 
 config PPC_4K_PAGES
 	bool "4k page size"
-	select HAVE_ARCH_SOFT_DIRTY if CHECKPOINT_RESTORE && PPC_BOOK3S
+	select HAVE_ARCH_SOFT_DIRTY if CHECKPOINT_RESTORE && PPC_BOOK3S_64
 
 config PPC_16K_PAGES
 	bool "16k page size"
@@ -566,7 +566,7 @@ config PPC_16K_PAGES
 config PPC_64K_PAGES
 	bool "64k page size"
 	depends on !PPC_FSL_BOOK3E && (44x || PPC_STD_MMU_64 || PPC_BOOK3E_64)
-	select HAVE_ARCH_SOFT_DIRTY if CHECKPOINT_RESTORE && PPC_BOOK3S
+	select HAVE_ARCH_SOFT_DIRTY if CHECKPOINT_RESTORE && PPC_BOOK3S_64
 
 config PPC_256K_PAGES
 	bool "256k page size"
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