powerpc: support for latencytop

Implement save_stack_trace_tsk on powerpc, so that we can run with
latencytop.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index aaf9989..5e21854 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -53,6 +53,9 @@
 	bool
 	default y
 
+config HAVE_LATENCYTOP_SUPPORT
+	def_bool y
+
 config TRACE_IRQFLAGS_SUPPORT
 	bool
 	depends on PPC64