clocksource/drivers/c-sky: fixup ftrace call-graph panic

We must add notrace on sched_clock_read, because it's called
by ftrace_graph_caller.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>

drivers/clocksource/timer-mp-csky.c

@@ -97,7 +97,7 @@ static int csky_mptimer_dying_cpu(unsigned int cpu)
 /*
  * clock source
  */
-static u64 sched_clock_read(void)
+static u64 notrace sched_clock_read(void)
 {
 	return (u64)mfcr(PTIM_CCVR);
 }