mediatek: add support for Airpi AP3000M

mediatek: add support for Airpi AP3000M

This adds support for the Airpi AP3000M 5G CPE based on MediaTek MT7981B.

Specifications:

SoC: MediaTek MT7981B (dual-core ARM Cortex-A53)
RAM: 1GB DDR4
Storage: 8GB or 16GB eMMC
Ethernet: 1x 2.5GbE LAN, 1x 1GbE WAN
Wireless: 2.4GHz/5GHz 802.11ax
Buttons: Reset, WPS
LEDs: System, 2.4GHz WiFi, 5GHz WiFi
Cooling: PWM-controlled fan
Expansion: M.2 slot for 5G modem

The factory partition (mmcblk0p2) is empty in stock firmware.
Execute via SSH before flashing OpenWrt:
```
dd if=/lib/firmware/MT7981_iPAiLNA_EEPROM.bin of=/dev/mmcblk0p2 bs=4k count=1
MAC addresses
```
Fixed MAC addresses are derived by macaddr_generate_from_mmc_cid.

Installation via U-Boot web page

Set static IP 192.168.88.2/255.255.255.0 on your computer.
Connect to the 1GbE port (WAN) and hold the reset button while booting the device. Wait for about 6 seconds, and release the reset button.
Open U-boot web page on your browser at http://192.168.88.1
Select the OpenWRT sysupgrade image, upload it, and start the upgrade.
Wait for automatic reboot.
Installation via sysupgrade

Flash the sysupgrade file via LuCI upgrade page without saving the settings.

Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21397
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>