ramips: add label MAC address for Onion Omega 2(+)

ramips: add label MAC address for Onion Omega 2(+)

Thanks to a hint from Michael Siegenthaler in 4b4fa2f9fed7 ("ramips:
fix ethernet MAC address on Omega2"), the label MAC address of
the Onion Omega 2(+) can be set based on its documentation [1].

[1] https://docs.onion.io/omega2-docs/mac-address.html

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>