ARM: cpuidle: Refactor rollback operations if init fails

If init fails, we need execute two levels rollback operations: the first
level is for the failed CPU rollback operations, the second level is to
iterate all succeeded CPUs to cancel their registration; currently the
code uses one function to finish these two levels rollback operations.

This commit is to refactor rollback operations, so it adds a new
function arm_idle_init_cpu() to encapsulate one specified CPU driver
registration and rollback the first level operations; and use function
arm_idle_init() to iterate all CPUs and finish the second level's
rollback operations.

Suggested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>