clocksource/drivers/vf_pit_timer: Prevent ftrace recursion

Having a traceable function in the sched_clock() path leads to a recursion
within ftrace and a kernel crash.

We should not trace the pit_read_sched_clock() function. Fix this by adding a
notrace attribute to this function.
Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/vf_pit_timer.c

@@ -52,7 +52,7 @@ static inline void pit_irq_acknowledge(void)
 	__raw_writel(PITTFLG_TIF, clkevt_base + PITTFLG);
 }
 
-static u64 pit_read_sched_clock(void)
+static u64 notrace pit_read_sched_clock(void)
 {
 	return ~__raw_readl(clksrc_base + PITCVAL);
 }