mediatek: filogic: add Unielec U7981-01 support

mediatek: filogic: add Unielec U7981-01 support

Hardware specification:
  SoC: MediaTek MT7981B 2x A53
  Flash: 8GB eMMC or 128 MB SPI-NAND
  RAM: 256MB
  Ethernet: 5x 10/100/1000 Mbps
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset
  USB: M.2(B-key) for 4G/5G Module
  Power: DC 12V 1A
  UART: 3.3v, 115200n8
  --------------------------
  |         Layout         |
  |   -----------------    |
  | 4 | VCC RX TX GND | <= |
  |   -----------------    |
  --------------------------

The U-boot menu will automatically appear at startup, and then select
the required options through UP/DOWN Key.

NAND Flash and eMMC Flash instructions:
1. Set your computers IP adress to 192.168.1.2.
2. Run a TFTP server providing the sysupgrade.bin image.
3. Power on the router, into the U-Boot menu.
4. Select "2. Upgrade firmware"
5. Update sysupgrade.bin file name, input server IP and input device
   IP (if they deviate from the defaults)
6. Wait for automatic startup after burning

Signed-off-by: Allen Zhao <allenzhao@unielecinc.com>