ramips: add support for Xiaomi R4AC v2 (intl)

ramips: add support for Xiaomi R4AC v2 (intl)

The second edition of international version of Mi Router 4A 100M is
very similar to the non-international one, but has another wireless chip.

 Installation
--------------

1. Initialize build-in firmware (use webgui for 192.168.31.1)
  You should install root password

2. Run OpenWRTInvasion for the first time (probably it will fail)
  Version 0.0.10 is working as well as 0.0.1.

3. Run OpenWRTInvasion for the second time
  It will create an access to your router

4. Upload sysupgrade image to router (/tmp/fw.bin)
  pc# nc -l 8080 < …/ramips/mt76x8/…-100m-intl-v2-squashfs-sysupgrade.bin
  router# nc 192.168.31.175 8080 > /tmp/fw.bin

5. Flash new firmware
  router# run mtd -r write /tmp/fw.bin OS1

6. Check result
  Wait about 5-10 minutes after flash. Router should reboot itself and
  turn left led from orange to blue.

In case of failure one can use Xiaomi 4a 100m debrick tool
(it uploads special image via tftpd in recovery mode)
After that you can start again from step 1.

Another actions are very similar to original Mi Router 4A 100M

 Original mtd paritions:
-------------------------

```
Creating 9 MTD partitions on "raspi":
0x000000000000-0x000001000000 : "ALL"
0x000000000000-0x000000020000 : "Bootloader"
0x000000020000-0x000000030000 : "Config"
0x000000030000-0x000000040000 : "Factory"
0x000000040000-0x000000050000 : "crash"
0x000000050000-0x000000060000 : "cfg_bak"
0x000000060000-0x000000160000 : "overlay"
0x000000160000-0x000000dc0000 : "OS1"
0x000000dc0000-0x000001000000 : "disk"
with special sub-partition
0x0000002c0000-0x000000dc0000 : "rootfs"
```

We will use OS1+disk space:
```
0x000000160000-0x000001000000 : "firmware"
```

Co-authored-by: Nita Vesa <nita.vesa@elektrik.link>
Signed-off-by: Anton Stratonnikov <billic@yandex.ru>
Link: https://github.com/openwrt/openwrt/pull/14304
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>