ath79: add support for UniFi AC-LR

The Unifi AC-LR has identical hardware to the Unifi AC-Lite.
The antenna setup is different according to the vendor,
which explains the thicker enclosure.

Therefore, it is helpful to know the exact device variant,
instead of having "Ubiquiti UniFi-AC-LITE/LR".

Signed-off-by: Andreas Ziegler <dev@andreas-ziegler.de>
[fix legacy name in commit message; add old boardname to
SUPPORTED_DEVICES]
Signed-off-by: David Bauer <mail@david-bauer.net>