cpufreq: Use consistent prefixing via pr_fmt

Use the more common kernel style adding a define for pr_fmt.

Miscellanea:

o Remove now unused PFX defines

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>