ramips: add support for D-Link DIR-2640 A1

ramips: add support for D-Link DIR-2640 A1

This patch adds support for D-Link DIR-2640 A1.

Specifications:
* Board: AP-MTKH7-0002
* 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 (blue/orange), Internet (blue/orange), WiFi 2.4G (blue),
        WiFi 5G (blue), USB 3.0 (blue), USB 2.0 (blue)

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 "firmware=@file.bin" 192.168.0.1

MAC addresses:

lan   factory 0xe000     *:a7 (label)
wan   factory 0xe006     *:aa
2.4   factory 0xe000 +1  *:a8
5.0   factory 0xe000 +2  *:a9

Seems like vendor didn't replace the dummy entries in the calibration data.

Signed-off-by: James McGuire <jamesm51@gmail.com>
[fix device definition title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>