ramips: add support for D-Link DWR-118-A2

ramips: add support for D-Link DWR-118-A2

The DWR-118-A2 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 (MT7612EN)
- 4x 10/100 Mbps Ethernet (1 WAN and 3 LAN)
- 1x 10/100/1000 Mbps Marvell Ethernet PHY (1 LAN)
- 2x external, non-detachable antennas
- 1x USB 2.0
- UART (J1) header on PCB (57600 8n1)
- 7x LED (5x GPIO-controlled), 2x button
- JBOOT bootloader

Known issues:
- GELAN not working
- flash is very slow

The status led has been assigned to the dwr-118-a2:green:internet led.
At the end of the boot it is switched off and is available for other
operation. Work correctly also during sysupgrade operation.

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: Cezary Jackiewicz <cezary@eko.one.pl>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>