thermal: core: Relocate functions that update trip points

In preparation for subsequent changes, move two functions used
for updating trip points, thermal_zone_set_trip_temp() and
thermal_zone_set_trip_hyst(), to thermal_core.c.

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/3248558.5fSG56mABF@rjwysocki.net