ramips: add support for D-Link DIR-867/DIR-882 A1

ramips: add support for D-Link DIR-867/DIR-882 A1

This patch adds support for D-Link DIR-867 A1 and D-Link DIR-882 A1. Given
the similarity of these devices, this patch also introduces a common DTS
shared between DIR-867 A1, DIR-878 A1 and DIR-882 A1.

Specifications:
* Board: AP-MTKH7-0002
* SoC: MediaTek MT7621AT
* RAM: 128 MB (DDR3)
* Flash: 16 MB (SPI NOR)
* WiFi: MediaTek MT7615N (x2)
* Switch: 1 WAN, 4 LAN (Gigabit)
* Ports: 1 USB 2.0, 1 USB 3.0
* Buttons: Reset, WiFi Toggle, WPS
* LEDs: Power (green/orange), Internet (green/orange), WiFi 2.4G (green),
        WiFi 5G (green), USB 2.0 (green), USB 3.0 (green)

Notes:
* WiFi 2.4G and WiFi 5G LEDs are wired directly to the wireless chips
* DIR-867 wireless chips are limited to 3x3 streams at hardware level
* USB ports and related LEDs available only on DIR-882

Serial port:
* Parameters: 57600, 8N1
* Location: J1 header (close to the Reset, WiFi and WPS buttons)
* Pinout: 1 - VCC
          2 - RXD
          3 - TXD
          4 - GND

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

Signed-off-by: Mateus B. Cassiano <mbc07@live.com>
[move DEVICE_VARIANT to individual definitions]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>