clocksource: exynos_mct: Use readl_relaxed/writel_relaxed

Using the __raw functions is discouraged.  Update the file to
consistently use the proper functions.

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