Thermal: rename thermal_sys.c to thermal_core.c

this is the preparation work to build all the thermal core framework
source file, like governors, cpu cooling, etc, into one module.

No functional change in this patch.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Acked-by: Durgadoss R <durgadoss.r@intel.com>