thermal: core: Add and use a reverse thermal zone guard

Add a guard for unlocking a locked thermal zone temporarily and use it
in thermal_zone_pm_prepare().

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/3344086.aeNJFYEL58@rjwysocki.net


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