ARM: EXYNOS4: Remove clock event timers using ARM private timers

External GIC cannot support PPI (Private Peripheral Interrupt) for
ARM private timers. Thus MCT should be selected as clock event timers
by default.

Signed-off-by: Changhwan Youn <chaos.youn@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>