mediatek: Add support for GL.iNet X3000 (Spitz AX) and XE3000 (Puli AX)

mediatek: Add support for GL.iNet X3000 (Spitz AX) and XE3000 (Puli AX)

The GL.iNet X3000 and XE3000 are Wi-Fi 6 5G cellular routers, based on
MediaTek MT7981A SoC. The XE3000 is the same device as the X3000,
except for an additional battery.

Specifications:
 - SoC: Filogic 820 MT7981A (1.3GHz)
 - RAM: DDR4 512M
 - Flash: eMMC 8G, MicroSD card slot
 - WiFi: 2.4GHz and 5GHz with 6 antennas
 - Ethernet:
   - 1x LAN (10/100/1000M)
   - 1x WAN (10/100/1000/2500M)
 - 5G: Quectel RM520N-GL with two nano-SIM card slots
 - USB: 1x USB 2.0 port
 - UART:
   - 3.3V, TX, RX, GND / 115200 8N1

MAC addresses as verified by OEM firmware:
vendor   OpenWrt    address     source
WAN      eth0       label       factory 0x0a (label)
LAN      eth1       label + 1
2g       phy0-ap0   label + 2   factory 0x04
5g       phy1-ap0   label + 3

Installation via U-Boot rescue:
1. Press and hold reset button while booting the device
2. Wait for the Internet led to blink 5 times
3. Release reset button
4. The rescue page is accessible via http://192.168.1.1
5. Select the OpenWrt sysupgrade image and start upgrade
6. Wait for the router to flash new firmware and reboot

Revert to stock firmware:
1. Download the stock firmware from GL.iNet website
2. Use the method explained above to flash the stock firmware

Switch the modem network port between PCIe and USB interfaces:
1. Connect to the AT commands (/dev/ttyUSB2) port using
e.g. minicom: minicom -D /dev/ttyUSB2
2. Check the current modem mode with 'AT+QCFG="data_interface"':
  - 0,0 indicates that the network port uses the USB interface
  - 1,0 indicates that the network port uses the PCIe interface
3. Switch the active interface with:
  - 'AT+QCFG="data_interface",0,0' to use the USB interface
  - 'AT+QCFG="data_interface",1,0' to use the PCIe interface
4. Reboot

Signed-off-by: Jean Thomas <jean.thomas@wifirst.fr>