clocksource/drivers/digicolor: 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 digicolor_timer_sched_read() function. Fix this by adding
the notrace attribute to this function.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>