Thermal: Add get trend, get instance API's to thermal_sys

This patch adds the following API's to thermal_sys.c, that
can be used by other Thermal drivers.
 * get_tz_trend: obtain the trend of the given thermal zone
 * get_thermal_instance: obtain the instance corresponding
   to the given tz, cdev and the trip point.

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