Thermal: thermal zone governor fix

This patch does a cleanup about the thermal zone govenor,
setting and make the following rule.
1. For thermal zone devices that are registered w/o tz->tzp,
   they can use the default thermal governor only.
2. For thermal zone devices w/ governor name specified in
   tz->tzp->governor_name, we will use the default govenor
   if the governor specified is not available at the moment,
   and update tz->governor when the matched governor is registered.

This also fixes a problem that OF registered thermal zones
are running with no governor.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Javi Merino <javi.merino@arm.com>