cpufreq: convert cpufreq_driver to using RCU

We eventually would like to remove the rwlock cpufreq_driver_lock or
convert it back to a spinlock and protect the read sections with RCU.
The first step in that direction is to make cpufreq_driver use RCU.
I don't see an easy wasy to protect the cpufreq_cpu_data structure
with RCU, so I am leaving it with the rwlock for now since under
certain configs __cpufreq_cpu_get is a hot spot with 256+ cores.

[rjw: Subject, changelog, white space]
Signed-off-by: Nathan Zimmer <[email protected]>
Acked-by: Viresh Kumar <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Nathan Zimmer