mediatek: add support for Ruijie RG-EW3200GX PRO

X32 Pro is another product  name for it in the Chinese market.

Specifications:
- SoC: MT7622B
- RAM: 256MB
- Flash: XMC XM25QH128C or Winbond WQ25Q128JVSQ 16MB SPI NOR
- Ethernet: 5x1GbE
- Switch: MT7531BE
- WiFi: 2.4G: MT7622 5G: MT7915AN+MT7975AN
- 3LEDs: System LED(blue) + Mesh LED(green) + Mesh LED(red)
- 2Keys: Mesh button + Reset button
- UART: Marked J19 on board. 3.3v, 115200n1
- Power: 12V 2.5A

MAC addresses as verified by OEM firmware:
use   address    source
WAN   *:F4       ethaddr@product_info
LAN   *:F5
5g    *:F6
2g    *:F7

Flash instruction:
1. Serve the initramfs.img using a TFTP server with address 10.10.10.3.
2. Interrupt the uboot startup process via UART.
3. Select "System Load Linux to SDRAM via TFTP" item.
4. (important) Back up firmware(mtd7) partitions with:
        dd if=/dev/mtd7 of=/tmp/firmware.bin
   and then download the firmware.bin image via SCP.
5. Flash the OpenWrt sysupgrade firmware.

Recovery stock firmware:
1. Transfer the firmware.bin image to the device.
2. Flash the image with:
        mtd write firmware.bin firmware

Signed-off-by: Langhua Ye <y1248289414@outlook.com>