ARM: EXYNOS: Change function name prefix for cpuidle

The driver was initially written for exynos4 but the driver is used
also for exynos5.

Change the function prefix name exynos4 -> exynos

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>