thermal: core: Rework .get_trend() thermal zone callback

Passing a struct thermal_trip pointer instead of a trip index to the
.get_trend() thermal zone callback allows one of its 2 implementations,
the thermal_get_trend() function in the ACPI thermal driver, to be
simplified quite a bit, and the other implementation of it in the
ti-soc-thermal driver does not even use the relevant callback argument.

For this reason, change the .get_trend() thermal zone callback
definition and adjust the related code accordingly.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>