ACPI: thinkpad-acpi: make EC-based thermal readings non-experimental

Reading the 16 thermal sensors directly from the EC has been stable for
about one year, in all supported ThinkPad models.  Remove its
"experimental" label.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>