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: I982f3b8fa6c58e259d8bfef0dfbd73df04555ca3