cpufreq: introduce hotplug governor

    The "hotplug" governor scales CPU frequency based on load, similar to
    "ondemand".  It scales up to the highest frequency when "up_threshold"
    is crossed and scales down one frequency at a time when "down_threshold"
    is crossed.  Unlike those governors, target frequencies are determined
    by directly accessing the CPUfreq frequency table, instead of taking
    some percentage of maximum available frequency.

    The key difference in the "hotplug" governor is that it will disable
    auxillary CPUs when the system is very idle, and enable them again once
    the system becomes busy.  This is achieved by averaging load over
    multiple sampling periods; if CPUs were online or offlined based on a
    single sampling period then thrashing will occur.

    Sysfs entries exist for "hotplug_in_sampling_periods" and for
    "hotplug_out_sampling_periods" which determine how many consecutive
    periods get averaged to determine if auxillery CPUs should be onlined or
    offlined.  Defaults are 5 periods and 20 periods respectively.
    Otherwise the standard sysfs entries you might find for "ondemand" and
    "conservative" governors are there.