ramips: add support for ipTIME AX2004M

ramips: add support for ipTIME AX2004M

ipTIME AX2004M is an 802.11ax (Wi-Fi 6) router, based on MediaTek
MT7621A.

Specification:
* SoC: MT7621A
* RAM: 256 MiB
* Flash: NAND 128 MiB
* Wi-Fi:
  * MT7915D: 2.4/5 GHz (DBDC)
* Ethernet: 5x 1GbE
  * Switch: SoC built-in
* USB: 1x 3.0
* UART: J4 (115200 baud)
  * Pinout: [3V3] (TXD) (RXD) (GND)

MAC address:

| interface |        MAC        |     source     | comment
|-----------|-------------------|----------------|---------
|       LAN | 58:XX:XX:00:XX:9B |                | [1]
|       WAN | 58:XX:XX:00:XX:99 |                |
|   WLAN 2G | 58:XX:XX:00:XX:98 | factory 0x4    |
|   WLAN 5G | 5A:XX:XX:40:XX:98 |                |
|           |                   |                |
|           | 58:XX:XX:00:XX:98 | config ethaddr |

[1] Used in this patch as WLAN 5G MAC address with the local bit set

Load address:
* stock
  * 0x80010000: FIT image
  * 0x81001000: kernel image -> entry
* OpenWrt
  * 0x80010000: FIT image
  * 0x82000000: uncompressed kernel+relocate image
  * 0x80001000: relocated kernel image -> entry

Installation via **recovery** mode:
1.  Press reset button, power up the device, wait >10s for CPU LED
    to stop blinking.
2.  Upload recovery image through the recovery web page at 192.168.0.1.

Revert to stock firmware:
1.  Install stock image via recovery mode.

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