ramips: add support for HUMAX E10

ramips: add support for HUMAX E10

HUMAX E10 (also known as HUMAX QUANTUM E10) is a 2.4/5GHz band AC router,
based on MediaTek MT7621A.

Specifications:
- SoC: MT7621A
- RAM: DDR3 128MB
- Flash: SPI NOR 16MB (MXIC MX25L12805D)
- WiFi:
  - 2.4GHz: MT7615
  - 5GHz: MT7615
- Ethernet: 2x 10/100/1000Mbps
  - Switch: SoC internal
- USB: 1x USB 2.0 Type-A
- UART: J1 (57600 8N1)
  - pinout: [3V3] (RXD) (GND) (TXD)

Installation via web interface:
- Flash **factory** image through the stock web interface.

Recovery procedure:
1. Connect ethernet cable between Router **LAN** 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...)
- It can accept both images which is
  HUMAX stock firmware dump (0x70000-0x1000000) image
  and OpenWRT **sysupgrade** image.

Signed-off-by: Kyoungkyu Park <choryu.park@choryu.space>
[remove trailing whitespace]
Signed-off-by: Sungbo Eo <mans0n@gorani.run>