clocksource: exynos_mct: register sched_clock callback

Use the clocksource mct-frc for sched_clock

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>