thermal: export weight to sysfs

It's useful to have access to the weights for the cooling devices for
thermal zones and change them if needed.  Export them to sysfs.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>