thermal: Constify the trip argument of the .get_trend() zone callback

Add 'const' to the definition of the 'trip' argument of the
.get_trend() thermal zone callback to indicate that the trip point
passed to it should not be modified by it and adjust the
callback functions implementing it, thermal_get_trend() in the
ACPI thermal driver and __ti_thermal_get_trend(), accordingly.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Michal Wilczynski <michal.wilczynski@intel.com>