cpufreq: Add helper to perform alloc/free of policy structure

Separate out the allocation of the cpufreq policy structure (along with
its error handling) to a helper function. This makes the code easier to
read and also helps with some upcoming code reorganization.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>