cpufreq: Move get_cpu_idle_time() to cpufreq.c

Governors other than ondemand and conservative can also use
get_cpu_idle_time() and they aren't required to compile
cpufreq_governor.c. So, move these independent routines to
cpufreq.c instead.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Git-commit: 72a4ce340a7ebf39e1c6fdc8f5feb4f974d6c635
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
[junjiew@codeaurora.org: update non-upstream files]
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>

Change-Id: I3227454599250011a19a752c962e3cacb1fb0301