cpufreq: CPPC: Add cppc_cpufreq_search_cpu_data

cppc_cpufreq_get_cpu_data() allocates a new struct cppc_cpudata
for the input CPU at each call.

To search the struct associated with a cpu without allocating
a new one, add cppc_cpufreq_search_cpu_data().
Also add an early prototype.

This will be used in a later patch, when generating artificial
performance states to register an artificial Energy Model in the
cppc_cpufreq driver and enable the Energy Aware Scheduler for ACPI
based systems.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>