ramips: add support for Rostelecom RT-SF-1

ramips: add support for Rostelecom RT-SF-1

Rostelecom RT-SF-1 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
ZigBee: 3.0, EFR32 MG1B232GG
Button: 2 buttons (Reset & WPS)
LEDs:
   - 1x Status (RGB)
   - 1x 2.4G (blue, hardware, mt76-phy0)
   - 1x 5G (blue, hardware, mt76-phy1)
Power: 12 VDC, 1.5 A
Connector type: barrel
Bootloader: U-Boot

Installation
-----------------
1. Remove dots from the OpenWrt factory image filename
2. Login to the router web interface
3. Update firmware using web interface with the OpenWrt factory image
4. If OpenWrt is booted, then no further steps are required. Enjoy!
   Otherwise (Stock firmware has booted again) proceed to the next step.
5. Update firmware using web interface with any version of the Stock
   firmware
6. Update firmware using web interface with the OpenWrt factory image

Revert to stock
---------------
Change bootflag to Sercomm1 in OpenWrt CLI and then reboot:
    printf 1 | dd bs=1 seek=7 count=1 of=/dev/mtdblock3

Recovery
--------
Use sercomm-recovery tool.
Link: https://github.com/danitool/sercomm-recovery
MAC Addresses
-------------
+-----+------------+------------+
| use | address    | example    |
+-----+------------+------------+
| LAN | label      | *:72, *:d2 |
| WAN | label + 11 | *:7d, *:dd |
| 2g  | label + 2  | *:74, *:d4 |
| 5g  | label + 3  | *:75, *:d5 |
+-----+------------+------------+
The label MAC address was found in Factory 0x21000

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>