examples/gnrc_networking_mac: whitelist ATmega256RFR2 based boards

[benpicco]

Contribution description

The example now works successfully on these AVR based boards.

Testing procedure

avr-rss2 can ping samr21-xpro

2021-02-20 00:41:33,491 # ping fe80::204:2519:1801:c905
2021-02-20 00:41:33,656 # 12 bytes from fe80::204:2519:1801:c905%7: icmp_seq=0 ttl=64 rssi=-33 dBm time=156.032 ms
2021-02-20 00:41:35,162 # 12 bytes from fe80::204:2519:1801:c905%7: icmp_seq=1 ttl=64 rssi=-38 dBm time=661.936 ms
2021-02-20 00:41:35,887 # 12 bytes from fe80::204:2519:1801:c905%7: icmp_seq=2 ttl=64 rssi=-37 dBm time=388.336 ms
2021-02-20 00:41:35,887 # 
2021-02-20 00:41:35,893 # --- fe80::204:2519:1801:c905 PING statistics ---
2021-02-20 00:41:35,898 # 3 packets transmitted, 3 packets received, 0% packet loss

Issues/PRs references

#16038

[aabadie]

LGTM

ACK!