drivers: cpu_cooling: Add core isolation as the last mitigation level

When the CPUs are extremely hot, scaling frequency to the least in the
frequency table may not be sufficient to prevent a thermal runaway
condition and preserve junction integrity. The only safe option is to
isolate the core from the runnable set of CPUs and let the CPU cool
down, before tasking the CPU again.

Core-isolation is a scheduler feature that migrates the load off the CPU
and isolates the core from the runnable list. This is a faster
alternative to hotlpug.

Use Core-isolation as the last level of CPU cooling. CPU cooling device
should be careful when specifying the mitigation levels in the DT,
especially since core-isolation may happen if allowed even when the trip
temperature for this drastic mitigation is not even hit, resulting in
performance impact.

Change-Id: I7e2bb9cb35a16c9c3651ee432d0d74f396e17d55
Signed-off-by: Lina Iyer <ilina@codeaurora.org>