[sonic_api] Implement function to get critical thermal value

device/celestica/x86_64-cel_questone2bd-r0/sonic_platform/thermal.py

@@ -170,6 +170,26 @@ def set_low_threshold(self, temperature):
                 break
         return result
 
+    def get_high_critical_threshold(self):
+        """
+        Retrieves the high critical threshold temperature of thermal
+        Returns:
+            A float number, the high critical threshold temperature of thermal in Celsius
+            up to nearest thousandth of one degree Celsius, e.g. 30.125
+        """
+        temp_file = "temp{}_crit".format(self.ss_index)
+        return self.__get_temp(temp_file)
+
+    def get_low_critical_threshold(self):
+        """
+        Retrieves the low critical threshold temperature of thermal
+        Returns:
+            A float number, the low critical threshold temperature of thermal in Celsius
+            up to nearest thousandth of one degree Celsius, e.g. 30.125
+        """
+        temp_file = "temp{}_lcrit".format(self.ss_index)
+        return self.__get_temp(temp_file)
+
     ##############################################################
     ###################### Device methods ########################
     ##############################################################