Thermal: Move thermal_instance to thermal_core.h

This patch creates a thermal_core.h file which can contain
all defines used by the core thermal framework files. For
now, move the thermal_instance structure to thermal_core.h
This structure is used by files under drivers/thermal/.

Signed-off-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>