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>

This fixes compiling errors without Ondemand

Based on https://patchwork.kernel.org/patch/2582231/