auto_hotplug: enable the min_online_cpus value

Use a module_param_cb to set the minimum number of online CPUs when the screen is on.

The values are between 1-4.

usage:
echo '1' < /sys/module/auto_hotplug/parameters/min_online_cpus
Revert "auto_hotplug: enable the min_online_cpus value"

This reverts commit c626c9856f0d125b7206ff9da50107f68ed18a19.
auto_hotplug.c: Add max_online_cpus() feature by motley
auto_hotplug.c: derp;;

Signed-off-by: Seongmin Park <r_data@naver.com>