arm: u8500_hotplug: various changes again

- use long instead of int for calculating current time in case of overflow
- tweak hotplugging queue delay
- add an offline timer so that cpu1 does not offline too quickly
- renamed stored time to hotplug time to avoid confusion
- added some comments for hotplug vars
- added some debugging messages
- only online/offline cpu1 if current governor is interactive
- some fixes for hotplug_time
- possibly less overhead

Change-Id: Ia8908b1ab94c69cf7c508788385a6604cf867646