ramips: add support for Xiaomi Mi Router 4AC (100M Edition)

[Sorin Pop]

- SoC:      MediaTek MT7628AN
- Flash:    16MB (Winbond W25Q128JV)
- RAM:      64MB
- Serial:   As marked on PCB, 3V3 logic, baudrate is 115200
- Ethernet: 3x 10/100 Mbps (switched, 2x LAN + WAN)
- WIFI0:    MT7628AN 2.4GHz 802.11b/g/n
- WIFI1:    MT7612EN 5GHz 802.11ac
- Antennas: 4x external (2 per radio), non-detachable
- LEDs:     Programmable power-LED (two-colored, yellow/blue)
            Non-programmable internet-LED (shows WAN-activity)
- Buttons:  Reset

INSTALLATION:

1. Connect to the serial port of the router and power it up.
   If you get a prompt asking for boot-mode, go to step 3.
2. Unplug the router after
       > Erasing SPI Flash...
       > raspi_erase: offs:20000 len:10000
   occurs on the serial port. Plug the router back in.
3. At the prompt select option 2 (Load system code then
   write to Flash via TFTP.)
4. Enter 192.168.1.1 as the device IP and 192.168.1.2 as the
   Server-IP.
5. Connect your computer to LAN1 and assign it as 192.168.1.2/24.
6. Serve the sysupgrade image via TFTP

This router seems to have the same hardware as Xiaomi Mi Router 4A
(100M Edition) but it uses a different U-Boot.

Signed-off-by: Sorin Pop <alexsorinpop at gmail.com>
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