arm: Add auto hotplug driver

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.