ramips: add support for Beeline SmartBox TURBO

ramips: add support for Beeline SmartBox TURBO

Beeline SmartBox TURBO is a wireless WiFi 5 router manufactured by
Sercomm company.

Device specification
--------------------
SoC Type: MediaTek MT7621AT
RAM: 256 MiB
Flash: 256 MiB, Micron MT29F2G08ABAGA3W
Wireless 2.4 GHz (MT7603EN): b/g/n, 2x2
Wireless 5 GHz (MT7615E): a/n/ac, 4x4
Ethernet: 5xGbE (WAN, LAN1, LAN2, LAN3, LAN4)
USB ports: 1xUSB3.0
Button: 2 buttons (Reset & WPS)
LEDs: 1 RGB LED
Power: 12 VDC, 1.5 A
Connector type: barrel
Bootloader: U-Boot

Installation
-----------------
1.  Login to the router web interface (admin:admin)
2.  Navigate to Settings -> WAN -> Add static IP interface (e.g.
    10.0.0.1/255.255.255.0)
3.  Navigate to Settings -> Remote cotrol -> Add SSH, port 22,
    10.0.0.0/255.255.255.0 and interface created before
4.  Change IP of your client to 10.0.0.2/255.255.255.0 and connect the
    ethernet cable to the WAN port of the router
5.  Connect to the router using SSH shell (SuperUser:SNxxxxxxxxxx, where
    SNxxxxxxxxxx is the serial number from the backplate label)
6.  Run in SSH shell:
       sh
7.  Make a mtd backup (optional, see related section)
8.  Change bootflag to Sercomm1 and reboot:
       printf 1 | dd bs=1 seek=7 count=1 of=/dev/mtdblock3
       reboot
9.  Login to the router web interface (admin:admin)
10. Remove dots from the OpenWrt factory image filename
11. Update firmware via web using OpenWrt factory image

Revert to stock
---------------
1. Change bootflag to Sercomm1 in OpenWrt CLI and then reboot:
      printf 1 | dd bs=1 seek=7 count=1 of=/dev/mtdblock3
2. Optional: Update with any stock (Beeline) firmware if you want to
   overwrite OpenWrt in Slot 0 completely.

mtd backup
----------
1. Set up a tftp server (e.g. tftpd64 for windows)
2. Connect to a router using SSH shell and run the following commands:
      cd /tmp
      for i in 0 1 2 3 4 5 6 7 8 9 10; do nanddump -f mtd$i /dev/mtd$i; \
      tftp -l mtd$i -p 10.0.0.2; md5sum mtd$i >> mtd.md5; rm mtd$i; done
      tftp -l mtd.md5 -p 10.0.0.2

MAC Addresses
-------------
+-----+-----------+---------+
| use | address   | example |
+-----+-----------+---------+
| LAN | label     | *:54    |
| WAN | label + 1 | *:55    |
| 2g  | label + 4 | *:58    |
| 5g  | label + 5 | *:59    |
+-----+-----------+---------+
The label MAC address was found in Factory 0x21000

Co-developed-by: Maximilian Weinmann <x1@disroot.org>
Signed-off-by: Maximilian Weinmann <x1@disroot.org>
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>