cppc_cpufreq: simplify use of performance capabilities

The CPPC performance capabilities are used significantly throughout
the driver.

Simplify the use of them by introducing a local pointer "caps" to
point to cpu_data->perf_caps, in functions that access performance
capabilities often.

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>