thermal: core: Store trip sysfs attributes in thermal_trip_desc

Instead of allocating memory for trip point sysfs attributes separately,
store them in struct thermal_trip_desc for each trip individually which
allows a few extra memory allocations to be avoided.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/46571375.fMDQidcC6G@rjwysocki.net