mediatek: add support for TOTOLINK A8000RU

Specifications:
- SoC: MT7622
- RAM: 512MB
- Flash: MX35LF1GE4AB 128MB SPI NAND
- Ethernet: RTL8367S 5x1GbE
- WiFi: 2.4G: MT7622 5G: MT7615N x2
- Other ports: USB3.0 x1

Flash instruction:
*important*: upgrade vendor firmware to at least V7.1cu.643_B20200521
1. hold the reset button and power on the device. wait for about 10s
   before releasing the reset button.
2. upload sysupgrade.bin via u-boot recovery page on http://192.168.1.1



Signed-off-by: Chuanhong Guo <gch981213@gmail.com>