powerpc: Move THREAD_SHIFT config to Kconfig

Shift the logic for defining THREAD_SHIFT logic to Kconfig in order to
allow override by users.
Signed-off-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Kconfig

@@ -678,6 +678,16 @@ config PPC_256K_PAGES
 
 endchoice
 
+config THREAD_SHIFT
+	int "Thread shift" if EXPERT
+	range 13 15
+	default "15" if PPC_256K_PAGES
+	default "14" if PPC64
+	default "13"
+	help
+	  Used to define the stack size. The default is almost always what you
+	  want. Only change this if you know what you are doing.
+
 config FORCE_MAX_ZONEORDER
 	int "Maximum zone order"
 	range 8 9 if PPC64 && PPC_64K_PAGES

arch/powerpc/include/asm/thread_info.h

@@ -10,15 +10,7 @@
 
 #ifdef __KERNEL__
 
-/* We have 8k stacks on ppc32 and 16k on ppc64 */
-
-#if defined(CONFIG_PPC64)
-#define THREAD_SHIFT		14
-#elif defined(CONFIG_PPC_256K_PAGES)
-#define THREAD_SHIFT		15
-#else
-#define THREAD_SHIFT		13
-#endif
+#define THREAD_SHIFT		CONFIG_THREAD_SHIFT
 
 #define THREAD_SIZE		(1 << THREAD_SHIFT)