ramips: add support for ipTIME A6ns-M

ramips: add support for ipTIME A6ns-M

ipTIME A6ns-M is a 2.4/5GHz band AC1900 router, based on MediaTek MT7621A.

Specifications:
- SoC: MT7621AT
- RAM: DDR3 128MB
- Flash: SPI NOR 16MB
- WiFi:
  - 2.4GHz: MT7615
  - 5GHz: MT7615
- Ethernet: 5x 10/100/1000Mbps
  - Switch: SoC internal
- UART:
  - J4: 3.3V, TX, RX, GND (3.3V is the square pad) / 57600 8N1

Installation via web interface:
1.  Flash **initramfs** image through the stock web interface.
2.  Boot into OpenWrt and perform sysupgrade with sysupgrade image.

Revert to stock firmware:
1.  Perform sysupgrade with stock image.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>