ramips: mt7620: Add support for D-Link DWR-961 A1

ramips: mt7620: Add support for D-Link DWR-961 A1

The DWR-961 A1 Wireless Router is based on the MT7620A SoC.
It's a merge of two Amit boards: DWR-960 with ethernet part
of Lava LR-25G001.

ROMID it's taken from Telenor branded version and it works with tested
device. Images from D-Link site for this router are from DWR-953 and it
have ROMID DLK6E2424001. I don't know if it's mistake on web-site
or if it's will require different image.

Specification:

- MediaTek MT7620A (580 Mhz)
- 128 MB of RAM
- 16 MB of FLASH
- 1x 802.11bgn radio
- 1x 802.11ac radio (MT7612 mpcie card)
- 5x 10/100/1000 Mbps Ethernet: 4xLAN and 1xWAN (QCA8337)
- 2x internal, non-detachable antennas (Wifi 2.4G)
- 3x external, detachable antennas (2x LTE, 1x Wifi 5G)
- 1x LTE modem cat 6
- UART (J5) header on PCB (57600 8n1)
- 13x LED, 2x button
- JBOOT bootloader

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>