thermal: core: use dev.groups to manage always present tz attributes

Thermal zones attributes are all being created using
device_create_file(). This has the disadvantage of making the code
complicated and sometimes we may miss the cleanup of them.

This patch starts to move the thermal zone sysfs attributes to the
dev.groups, so Linux device core manage them for us. For now, this patch
only moves those attributes are always present regardless of thermal
zone condition.

This change has also the advantage of cleaning up the thermal zone
parameters sysfs entries that are left unclean after device
registration.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>