How to Terminate DONE pin on TPL5110 when unused?
A valid DONE input is a signal rising from low to high. Tie the DONE pin to ground via 10k resistor.   

You also have to make sure the MCU pins are never configured to drive high if at all there is a connection from MCU output to the DONE pin. 


The DONE pulse width has to be a minimum of 100ns still. To be considered as valid input (from the datasheet). You can add a capacitor as well to filter out any transients on the line.