When multiple thermal zones are bound to the same cooling device, multiple
kernel threads may want to update the cooling device state by calling
thermal_cdev_update(). Having cdev not protected by a mutex can lead to a race
condition. Consider the following situation with two kernel threads k1 and k2:

	    Thread k1				Thread k2
                                    ||
                                    ||  call thermal_cdev_update()
                                    ||      ...
                                    ||      set_cur_state(cdev, target);
    call power_actor_set_power()    ||
        ...                         ||
        instance->target = state;   ||
        cdev->updated = false;      ||
                                    ||      cdev->updated = true;
                                    ||      // completes execution
    call thermal_cdev_update()      ||
        // cdev->updated == true    ||
        return;                     ||
                                    \/
                                    time

k2 has already looped through the thermal instances looking for the deepest
cooling device state and is preempted right before setting cdev->updated to
true. Now, k1 runs, modifies the thermal instance state and sets cdev->updated
to false. Then, k1 is preempted and k2 continues the execution by setting
cdev->updated to true, therefore preventing k1 from performing the update.
Notice that this is not an issue if k2 looks at the instance->target modified by
k1 "after" it is assigned by k1. In fact, in this case the update will happen
anyway and k1 can safely return immediately from thermal_cdev_update().

This may lead to a situation where a thermal governor never updates the cooling
device. For example, this is the case for the step_wise governor: when calling
the function thermal_zone_trip_update(), the governor may always get a new state
equal to the old one (which, however, wasn't notified to the cooling device) and
will therefore skip the update.

CC: Zhang Rui <rui.zhang@intel.com>
CC: Eduardo Valentin <edubezval@gmail.com>
CC: Peter Feuerer <peter@piie.net>
Reported-by: Toby Huang <toby.huang@arm.com>
Signed-off-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Reviewed-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>