mediatek: filogic: add support for ipTIME AX3000M

mediatek: filogic: add support for ipTIME AX3000M

ipTIME AX3000M is an 802.11ax (Wi-Fi 6) router, based on MediaTek
MT7981B. (Filogic 820)

Specifications:
* SoC: MetiaTek MT7981B
* RAM: 256MiB
* Flash: ESMT SPI-NAND F50L1G41LB 128MiB
* Wi-Fi:
  * MediaTek MT7915E: 2.4GHz and 5GHz
* Ethernet: 5x 1GbE
  * Switch: MediaTek MT7531
* USB: 1x 3.0
* UART: J4 (115200 baud)
* LED:
  * PWR: VCC
  * CPU, 2.4G, 5G: GPIO
  * LAN 1-4, WAN: Controlled by Switch

MAC Addresses:
* 2.4G, 5G: B0:XX:XX:04:2A:60 (factory 0x4)
* WAN: B0:XX:XX:04:2A:61 (factory 0x4, +1)
* LAN: B0:XX:XX:04:2A:63 (factory 0x4, +3)

MTD Partitions:
* 0x000000000000-0x000000100000 : "BL2"
* 0x000000100000-0x000000180000 : "u-boot-env"
* 0x000000180000-0x000000380000 : "Factory"
* 0x000000380000-0x000000580000 : "FIP"
* 0x000000580000-0x000007380000 : "ubi"

UBI Partitions (Dynamic):
* id: 0, kernel2
* id: 1, kernel
* id: 2, rootfs
* id: 3, rootfs_data
* id: 4, rootfs2

Installation:
* Upload factory image through the tftp recovery mode.

Notes:
* This device has a dual-boot partition scheme, if installing with the
stock web interface method will flash only on the inactive ubi partitions
which are kernel and rootfs, newly flashed kernel didn't know the proper
rootfs partition so the booted kernel will panic.

Tested-by: Rhnn Hur <hurrhnn@icmp.kr>
Signed-off-by: Rhnn Hur <hurrhnn@icmp.kr>
Link: https://github.com/openwrt/openwrt/pull/16643
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>