Thermal: move the KELVIN_TO_MILLICELSIUS macro to thermal.h

This macro can be used by other component so move it to a common header,
but in a slightly different way: define two macros, one macro with an
offset and the other doesn't.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>