ARM: EXYNOS: Use cpuidle_register

Use the cpuidle generic function 'cpuidle_register'. That saves us
from some extra lines of code and unneeded variables.

A side effect of this change is a bug fix where before the cpuidle
driver was registered for each_online_cpu and now it is for
each_possible_cpu.

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>