clocksource: exynos_mct: Register the timer for stable udelay

This patch registers the exynos mct clocksource as the current timer
as it has constant clock rate. This will generate correct udelay for
the exynos platform and avoid using unnecessary calibrated
jiffies. This change has been tested on exynos5420 based board and
udelay is very close to expected.

Without this patch udelay() on exynos5400 / exynos5800 is wildly
inaccurate due to big.LITTLE not adjusting loops_per_jiffy correctly.
Also without this patch udelay() on exynos5250 can be innacruate
during transitions between frequencies < 800 MHz (you'll go 200 MHz ->
800 MHz -> 300 MHz and will run at 800 MHz for a time with the wrong
loops_per_jiffy).

[dianders: reworked and created version 3]

Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>