ACPI: thermal: Determine the number of trip points earlier

Compute the number of trip points in acpi_thermal_add() so as to allow the
driver's data structures to be simplified going forward.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>