cpuidle: Make cpuidle governor switchable to be the default behaviour

For now cpuidle governor can be switched via sysfs only when the
boot option "cpuidle_sysfs_switch" is passed, but it's important
to switch the governor to adapt to different workloads, especially
after TEO and haltpoll governor were introduced.

Add available_governors and current_governor into the default
attributes, but reserve the current_governor_ro for compatiblity.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Doug Smythies <dsmythies@telus.net>
Tested-by: Doug Smythies <dsmythies@telus.net>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>