arm: Add auto hotplug driver (Again)

This driver is simple and cheap and aims to avoid unnecessary hotplug events and unnecessary task migrations. The design is simple. The value of nr_running() (which is the current number of runnable threads) is averaged over X samples by a ringbuffer. If the value is over a dynamically calculated value, all CPUs are onlined. When the value falls under another dynamically calculated value, the CPUs are offlined with a 2 second delay.

If there is persistent load on the system, the CPUs will not continually
thrash between onlining and offlining and continually migrate tasks.
The number of online CPUs will remain constant until the load has dropped
below the dynamically calculated value.

The online and offline values are dynamically scaled based on the number of
CPUs online, as is the sampling rate of the driver. This aims to reduce the
expense of the driver and also reduce the number of unless hotplug transitions
which could have been avoided had cpufreq been given the chance to ramp the
online CPUs up and deal with the load.

This driver will also offline secondary cores when the screen is turned off,
providing earlysuspend is available.
It also has a module param to set the minimum number of online CPUs:
/sys/module/auto_hotplug/parameters/min_online_cpus
This value is ignored when the device enters earlysuspend, as the secondary
CPUs are all offlined, but is set again when the screen is turned back on.