thermal: core: Manage thermal_governor_lock using a mutex guard

Switch over the thermal core to using a mutex guard for
thermal_governor_lock management.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/3679429.R56niFO833@rjwysocki.net


Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>