clocksource: exynos_mct: Fix ftrace

In (93bfb769

 clocksource: exynos_mct: register sched_clock callback) we
supported using the MCT as a scheduler clock.  We properly marked
exynos4_read_sched_clock() as notrace.  However, we then went and
called another function that _wasn't_ notrace.  That means if you do:

  cd /sys/kernel/debug/tracing/
  echo function_graph > current_tracer

You'll get a crash.

Fix this (but still let other readers of the MCT be trace-enabled) by
adding an extra function.  It's important to keep other users of MCT
traceable because the MCT is actually quite slow to access and we want
exynos4_frc_read() to show up in ftrace profiles if it's the
bottleneck.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>