thermal: core: introduce thermal_helpers.c

Here we have a simple code organization. This patch moves
functions that do not need to handle thermal core internal
data structure to thermal_helpers.c file.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>