thermal: core: Add new event for sending keep alive notifications

This event is sent by the platform firmware to confirm that
user space thermal solution is alive. The response to this event
from the user space thermal solution is platform specific.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200915223650.406046-3-srinivas.pandruvada@linux.intel.com