ramips: add support for ipTIME A3004NS-dual

ipTIME A3004NS-dual is a 2.4/5GHz band router, based on Mediatek MT7621.

Specifications:
- SoC: MT7621 (880MHz)
- RAM: DDR3 256M
- Flash: SPI NOR 16MB
- WiFi:
 - 2.4GHz: MT7602E
 - 5GHz : MT7612E
- Ethernet:
 - 4x LAN
 - 1x WAN
- USB: 1 * USB3.0 port
- UART:
  - 3.3V, TX, RX, GND / 57600 8N1

Installation via web interface:
- 1. Flash Initramfs image using OEM Firmware's web GUI
- 2. Boot into OpenWrt and perform Sysupgrade with sysupgrade image.

Revert to stock firmware:
- 1. Boot into OpenWrt and perform Sysupgrade with OEM Stock Firmware image.

Signed-off-by: Yuchan Seo <hexagonwin@disroot.org>
Reviewed-by: Sungbo Eo <mans0n@gorani.run>