cppc_cpufreq: clean up cpu, cpu_num and cpunum variable use

In order to maintain the typical naming convention in the cpufreq
framework:

 - replace the use of "cpu" variable name for cppc_cpudata pointers
   with "cpu_data"
 - replace variable names "cpu_num" and "cpunum" with "cpu"
 - make cpu variables unsigned int

Where pertinent, also move the initialisation of cpu_data variable to
its declaration and make consistent use of the local "cpu" variable.
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>