cpufreq: cpufreq-cpu0: Use generic cpufreq routines

Most of the CPUFreq drivers do similar things in .exit() and .verify() routines
and .attr. So its better if we have generic routines for them which can be used
by cpufreq drivers then.

This patch uses these generic routines in the cpufreq-cpu0 driver.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>