From the data sheet
AD15 is not an actual pin but is connected to a temperature sensor.
and
The temperature sensor is connected to Channel 15 of the ADC.
The temperature sensor provides an output voltage VT that is proportional to absolute temperature (PTAT). To activate the temperature sensor, TSON bit (ADC_ACR) needs to be set.
So analogRead(15) will only work if the TSON bit in ADC_ACR is set by the looks of it. So it's up to the Arduino guys to do that or I guess you can do it yourself.
Rob