drivers: thermal: cpu_cooling: allow platform freq mitigation

cpu device can be controlled by a hardware platform device and in those
cases the cpu cooling device interface should communicate with the
platform device instead of the cpufreq module.

Allow platform drivers to register with CPU cooling with their frequency
mitigation functions. This allows the cpu cooling interface to
communicate the frequency mitigations to the platform driver directly.

Change-Id: I47960b002bf1bce1cd588de2892de46793a95562
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>