ramips: add support for ipTIME A6004NS-M

ipTIME A6004NS-M is a 2.4/5GHz band AC1900 router, based on MediaTek MT7621A.

Specifications:
- SoC: MediaTek MT7621A (880MHz, Duel-Core)
- RAM: DDR3 256MB
- Flash: SPI NOR 16MB (Winbond W25Q128BV)
- WiFi: MediaTek MT7615E (2.4GHz, 5GHz)
- Ethernet: MediaTek MT7530 (WAN x1, LAN x4, SoC built-in Estimated)
- USB: USB 3.0 x1
- UART: [3.3V, TX, RX, GND] (57600 8N1)

Installation via web interface:
1. Flash initramfs image using OEM's Firmware Update page.
2. Connect to OpenWrt with an SSH connection to `192.168.1.1`.
3. Perform sysupgrade with sysupgrade image.

Revert to stock firmware:
1.  Flash stock firmware via OEM's Recovery mode

How to use OEM's Recovery mode:
1. Power on the device and connect the shell through UART.
2. Connect to the shell and press the `t` key on the keyboard.
3. Set fixed IP with  `192.168.0.2`  with subnet mask  `255.255.255.0`
4. Flash image via TFTP to  `192.168.0.1`

Additional Notes:
1. The higher the 5Ghz Frequency, the lower the stability. It is recommended to use less than 5.775Ghz.
2. If the 5Ghz frequency is too high, 5Ghz may not work.
3. A6ns-M use shared dtsi file of A6004NS-M. (reference: /mt7621_iptime_a6004ns-m.dtsi).

Signed-off-by: SeongUk Moon <antegral@antegral.net>
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Signed-off-by: Sungbo Eo <mans0n@gorani.run>

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