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.

Specifications:
* 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 addresses:

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

Notes:
* This device has a dual-boot partition scheme, but this firmware works
  only on boot partition 1. The stock web interface will flash only on the
  inactive boot partition, but the recovery web page will always flash on
  boot partition 1.

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>