powerpc: expose number of page table levels on Kconfig level

We would want to use number of page table level to define mm_struct.
Let's expose it as CONFIG_PGTABLE_LEVELS.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/powerpc/Kconfig

@@ -297,6 +297,12 @@ config ZONE_DMA32
 	bool
 	default y if PPC64
 
+config PGTABLE_LEVELS
+	int
+	default 2 if !PPC64
+	default 3 if PPC_64K_PAGES
+	default 4
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"