csky: basic ftrace supported

When gcc with -pg, it'll add _mcount stub in every function. We need
implement the _mcount in kernel and ftrace depends on stackstrace.

To do: call-graph, dynamic ftrace

Signed-off-by: Guo Ren <ren_guo@c-sky.com>

arch/csky/include/asm/ftrace.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+// Copyright (C) 2018 Hangzhou C-SKY Microsystems co.,ltd.
+
+#ifndef __ASM_CSKY_FTRACE_H
+#define __ASM_CSKY_FTRACE_H
+
+extern void _mcount(unsigned long from_pc);
+
+#endif /* __ASM_CSKY_FTRACE_H */