CC2520/ATtiny861A pin configuration
From the CC2520 datasheet it looks like all you need to do is control the resetn of the CC2520 with any GPIO pin. As long as you can drive the pin high and low you should be fine. Any of those pins should work provided they are configured as outputs. 

I might consider adding an external pullup resistor on that resetn line if you don't want the CC2520 to be reset when the MCU gets reset, otherwise it shouldn't really matter.