clocksource/drivers/exynos_mct: Staticize struct clocksource

The struct clocksource 'mct_frc' is not exported and used outside so
make it static.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>