ACPI: thermal: Use library functions to obtain trip point temperature values

Modify the ACPI thermal driver to use functions from the ACPI thermal
library to obtain trip point temperature values instead of duplicating
them locally.

Among other things, this requires the functions in question to be
exported to it, because it can be built as a module.

It effectively changes the behavior of the driver to treat temperature
values out of the reasonable range (-55 centigrade to 175 centigrade) as
invalid, but there is no other expected functional impact.

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