thermal: core: Separate code running under thermal_list_lock

thermal: core: Separate code running under thermal_list_lock

To prepare for a subsequent change that will switch over the thermal
core to using a mutex guard for thermal_list_lock management, move the
code running under thermal_list_lock during the initialization and
unregistration of cooling devices into separate functions.

While at it, drop some comments that do not add value.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/10572828.nUPlyArG6x@rjwysocki.net
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>