ramips: add support for D-Link DWR-960

ramips: add support for D-Link DWR-960

The DWR-960 Wireless Router is based on the MT7620A SoC.

Specification:

- MediaTek MT7620A (580 Mhz)
- 128 MB of RAM
- 16 MB of FLASH
- 1x 802.11bgn radio
- 1x 802.11ac radio (MT7610 mpcie card)
- 4x 10/100 Mbps Ethernet (1 WAN and 3 LAN)
- 1x 10/100/1000 Mbps Ethernet (1 LAN) (AR8035)
- 2x internal, non-detachable antennas (Wifi 2.4G)
- 3x external, detachable antennas (2x LTE, 1x Wifi 5G)
- 1x LTE modem
- UART (J4) header on PCB (57600 8n1)
- 9x LED, 2x button
- JBOOT bootloader

Known issues:
- Flash is extremely slow.

Installation:
Apply factory image via http web-gui or JBOOT recovery page

How to revert to OEM firmware:
- push the reset button and turn on the power. Wait until LED start
  blinking (~10sec.)
- upload original factory image via JBOOT http (IP: 192.168.123.254)

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>