Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Provide a new CPU device driver ivar to report the nominal speed of the
CPU, if available. This is meant to solve the issue of cpufreq misreporting
speeds on CPUs that boot in a reduced power mode and have only relative
speed control.

Add an interface for drivers to be notified of changes to CPU frequency.
cpufreq_pre_change is called before the change, giving each driver a chance
to revoke the change. cpufreq_post_change provides the results of the
change (success or failure). cpufreq_levels_changed gives the unit number
of the cpufreq device whose number of available levels has changed. Hook
in all the drivers I could find that needed it.

* TSC: update TSC frequency value. When the available levels change, take the
highest possible level and notify the timecounter set_cputicker() of that
freq. This gets rid of the "calcru: runtime went backwards" messages.
* identcpu: updates the sysctl hw.clockrate value
* Profiling: if profiling is active when the clock changes, let the user
know the results may be inaccurate.

Reviewed by: bde, phk
MFC after: 1 month

Add a new field to struct cf_setting for special values. These are driver-
specific values that other components may want to use. Add support to
acpi_perf(4) to export the control and status values via this field.

Add the new CPUFREQ_FLAG_INFO_ONLY flag for drivers which can attach to
read info about the possible settings for a CPU but cannot control them
directly.

Add a new include to handle generic CPU interfaces.