thermal: core: Rename trip list node in struct thermal_trip_desc

Since the list node field in struct thermal_trip_desc is going to be
used for purposes other than trip crossing notification, rename it
to list_node.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/2201558.irdbgypaU6@rjwysocki.net