octeon: add support for Ubiquiti UniFi Security Gateway

octeon: add support for Ubiquiti UniFi Security Gateway

Ubiquiti UniFi Security Gateway (USG) is largely identical to
the EdgeRouter Lite (ERLite-3) apart from a different board ID
and two dome leds.

Device data (from WikiDev):
  CPU: Cavium Octeon Plus CN5020 @500MHz 2-cores
  Ethernet: 3x Atheros AR8035-A GbE PHY's
  Flash: On-board 4MB Flash
  Storage: Internal 3.8GB USB Flash (Kingston ID) drive
    w/ 1.5GB free for use occupies single internal USB port.
  Serial: 1x RJ45 port on front panel. 115200, 8N1
  Buttons: 1x Reset

Flash instructions are identical to EdgeRouter Lite.

Signed-off-by: Clemens Hopfer <openwrt@wireloss.net>