clocksource: exynos_mct: use (request/free)_irq calls for local timer registration

Replace the (setup/remove)_irq calls for local timer registration with
(request/free)_irq calls. This generalizes the local timer registration API.
Suggested by Mark Rutland.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>