thermal: add the support for building the generic thermal as a module

Build the generic thermal driver as module "thermal_sys".

Make ACPI thermal, video, processor and fan SELECT the generic
thermal driver, as these drivers rely on it to build the sysfs I/F.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Len Brown <len.brown@intel.com>