performance: Add comments for CPU FREQ usage

Add comments to inform users on the CPU
frequencies PerfLock supports and usage.

Change-Id: I0c60865d320dfc4e70d38b179831636944fd016c
(cherry picked from commit 112837d983602dfa8f50f10427cc9805dd2c1e01)
(cherry picked from commit d577225e8a4136363a017aae23cc49a9b3bc660b)
(cherry picked from commit f78dfdbac29ea38ed228a9eb101fb7416ceda783)