ramips: add support for HUMAX E2

ramips: add support for HUMAX E2

HUMAX E2 (also known as HUMAX QUANTUM E2) is a 2.4/5GHz band AC router,
based on MediaTek MT7620A.

Specifications:
- SoC: MT7620A
- RAM: DDR2 64MB
- Flash: SPI NOR 8MB (MXIC MX25L6405D)
- WiFi:
  - 2.4GHz: SoC internal
  - 5GHz: MT7610E
- Ethernet: 1x 10/100Mbps
  - Switch: SoC internal
- UART: J2 (57600 8N1)
  - pinout: [3V3] (RXD) (GND) (TXD)

Installation and Recovery via TFTP:
1. Connect ethernet cable between Router port and PC Ethernet port.
2. Set your computer to a static IP **192.168.1.1**
3. Turn the device off and wait a few seconds. Hold the WPS button on front
   of device and insert power.
4. Send a firmware image to **192.168.1.6** using TFTP.
   You can use any TFTP client. (tftp, curl, Tftpd64...)
5. Wait until Power LED stop flashing. **DO NOT TURN OFF DEVICE!**
   The device will be automatically rebooted.

Signed-off-by: Kyoungkyu Park <choryu.park@choryu.space>