ramips: add support for D-Link DIR-2660 A2

ramips: add support for D-Link DIR-2660 A2

This patch adds support for D-Link DIR-2660 A2.
Based on the commit: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=b5dd746cbb1aaf91f4b68e9f3eda97413550d904,

Both devices look identical, except for the A1/A2 designation.
You can safely install the A1 firmware for the A2 – I've been testing it for several months as a DUMB AP – without any problems.

Specifications:
* SoC: MediaTek MT7621AT
* RAM: 256 MB (DDR3)
* Flash: 128 MB (NAND)
* WiFi: MediaTek MT7615N (x2)
* Switch: 1 WAN, 4 LAN (Gigabit)
* Ports: 1 USB 2.0, 1 USB 3.0
* Buttons: Reset, WPS
* LEDs: Power (white/orange), Internet (white/orange), WiFi 2.4G (white),
        WiFi 5G (white), USB 3.0 (white), USB 2.0 (white)

Notes:
* WiFi 2.4G and WiFi 5G LEDs are wired directly to the wireless chips

Installation:
* D-Link Recovery GUI: power down the router, press and hold the reset
  button, then re-plug it. Keep the reset button pressed until the power
  LED starts flashing orange, manually assign a static IP address under
  the 192.168.0.xxx subnet (e.g. 192.168.0.2) and go to http://192.168.0.1

* Some modern browsers may have problems flashing via the Recovery GUI,
  if that occurs consider uploading the firmware through cURL:

    curl -v -i -F "[email protected]" 192.168.0.1

Signed-off-by: Marcin Leksmark <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20020
Signed-off-by: Hauke Mehrtens <[email protected]>