drivers: cpu_cooling: Save mitigation state for core isolation

CPU cooling device doesn't store the mitigation state when a core
isolation is requested. Hence the state variable will hold a no
mitigation state value. So when the mitigation is cleared, the CPU
cooling device assumes it is already in the no mitigation state and wont
clear the core isolation.

To avoid this, store the mitigation state during core isolation request.
Thus allowing the mitigation clear action to clear core isolation.

Change-Id: Ida48c4767e8d6123495fc5fff9a86183e85b8192
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>