clocksource: mct: use fin_pll clock as the tick clock source for mct

With the migration of Exynos4 clocks to use common clock framework, the
old styled 'xtal' clock is not used anymore. Instead, the clock 'fin_pll'
is used as the tick clock for mct controller.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>