ramips: add support for ipTIME A604M

ramips: add support for ipTIME A604M

ipTIME A604M is a 2.4/5GHz band AC1200 router, based on MediaTek
MT7628AN.

Specifications:
- SoC: MT7628AN
- RAM: DDR2 64MB
- Flash: SPI NOR 8MB
- WiFi:
  - 2.4GHz: SoC internal
  - 5GHz: MT7612EN
- Ethernet: 5x 10/100Mbps
  - Switch: SoC internal
- UART:
  - J1: 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>