cores: Fix PCINT8 for ATmega128RFA1 and ATmega128RFR2

PCINT8 was previously not setup. Per the spec sheet it is on Port E pin
0.
Merge pull request #73 from cardoe/rfa1-rfr2-pcint8

cores: Fix PCINT8 for ATmega128RFA1 and ATmega128RFR2