ath79: add support for D-Link DAP-2680 A1

ath79: add support for D-Link DAP-2680 A1

Specifications:
 * QCA9558, 16 MiB Flash, 256 MiB RAM, 802.11n 3T3R
 * QCA9984, 802.11ac Wave 2 3T3R
 * Gigabit LAN Port (AR8035), 802.11at PoE

Installation:
 * Factory Web UI is at 192.168.0.50
   login with 'admin' and blank password, flash factory.bin
 * Recovery Web UI is at 192.168.0.50
   connect network cable, hold reset button during power-on and keep it
   pressed until uploading has started (only required when checksum is ok,
   e.g. for reverting back to oem firmware), flash factory.bin

After flashing factory.bin, additional free space can be reclaimed by
flashing sysupgrade.bin, since the factory image requires some padding
to be accepted for upgrading via OEM Web UI.

Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>