Interfacing a temperature sensor with PLD without implementing SPI/I2C bus
There are PWM output temperature sensors.  However, this requires you to implement logic to count the ratio between the high and low time of the pulse and convert that into a temperature.  This requires an oscillator, counters, etc.

The logic to convert PWM back to a temperature would probably be less complicated than an I2C interface, but perhaps more complicated than a simple SPI interface.

The MAX6577 converts the temperature into a pulse frequency.  With a crystal oscillator that is much faster than the frequency period, you could count the number of clock periods in the temperature period and convert that without a ton of logic.  I think the MAX6577 would be your best bet.