ramips: add support for D-Link DIR-1960 A1

ramips: add support for D-Link DIR-1960 A1

This patch adds support for D-Link DIR-1960 A1. Given the similarity with
the DIR-1760/2660 A1, this patch also introduces a common DTSI which can
be shared with these devices, with support to be added in future commits.

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 3.0
* Buttons: Reset, WPS
* LEDs: Power (white/orange), Internet (white/orange), WiFi 2.4G (white),
        WiFi 5G (white), USB 3.0 (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 "firmware=@file.bin" 192.168.0.1

MAC addresses:

lan   factory 0xe000     *:EB (label)
wan   factory 0xe006     *:EE
2.4   factory 0xe000 +1  *:EC
5.0   factory 0xe000 +2  *:ED

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

Signed-off-by: Josh Bendavid <joshbendavid@gmail.com>
[fix whitespace issues, create patch to merge DIR-1960 first, move
special WiFi MAC settings to DTS, extend commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>