drivers: thermal: Aggregate userspace mitigation request

Thermal core allows multiple governors to monitor different thermal
zones. Each governor's mitigation request shouldn't override other
governor's request. But userspace request to mitigate a cooling device
will override any previous aggregated request from kernel thermal
governors. This will not allow any userspace governor to mitigate
along with the kernel governors.

The userspace request now will be stored and aggregated with the
rest of the requests from the kernel thermal governors.

Change-Id: Ib0d55a0ee7ad8f835a5e969b7ef268d420085acf
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>