drivers: cpu_cooling: allow platform freq mitigation

Allow platform drivers to register with CPU cooling with their frequency
mitigation functions. This allows the platform drivers to provide the
callbacks to get the current ceil limit and to set the floor and ceil
limits for the cpu.

Change-Id: I47960b002bf1bce1cd588de2892de46793a95562
Signed-off-by: Lina Iyer <ilina@codeaurora.org>