ramips: mt7621: add support for Keenetic Viva (KN-1910)

ramips: mt7621: add support for Keenetic Viva (KN-1910)

Specification:
SoC: MediaTek MT7621A
RAM: 128M DDR3, Winbond W631GG6MB-12 (DDR3-1600) or Winbond W631GG6MB-11
Flash: 128M, Macronix MX30LF1G18AC-TI (Dual Boot, Parallel-NAND)
Switch: MT7530, 5 ports 1Gbps
WiFi: MT7615DN, 2.4GHz 802.11n and 5GHz 802.11ac
USB: 2 ports USB 2.0
GPIO: 4 buttons (Wi-Fi, Reset, FN1, FN2), 4 LEDs (Power, Internet, FN, Wi-Fi), USB port power controls

LAN: RF-EEPROM + 0x04
WAN: RF-EEPROM + 0x28
2.4 GHz: RF-EEPROM + 0x04
5 GHz: 2.4GHz + 82:00:00:00:00:00

Disassembly:
There are 2 screws at the bottom. After removing the screws, pry the gray plastic part around (it is secured with latches) and remove it.

Serial Interface:
The serial interface can be connected to the 4 pin dots to the left of the radiator.
Pins (from LAN ports to LEDs):
3.3V (do not connect)
TX
RX
GND
Settings: 57600, 8N1

Flashing via OEM recovery software:
1. Download the OEM recovery software from the manufacturer's website
2. Download the firmware image (for OpenWRT it is *-squashfs-factory.bin), rename it to KN-1910_recovery.bin
3. Replace the file in the fw folder OEM recovery software with the file from step 2.
4. Run the OEM recovery software and follow the instructions.

Flashing via TFTP:
1. Connect your PC and router to port 1-4, configure PC interface using IP 192.168.1.2, mask 255.255.255.252
2. Serve the firmware image (for OpenWRT it is *-squashfs-factory.bin) renamed to KN-1910_recovery.bin via TFTP
3. Power up the router while pressing Reset button on the back
4. Release Restart button when Power LED starts blinking

To revert back to OEM firmware:
The return to the OEM firmware is carried out by using the methods described above with the help of the appropriate firmware image.

Keenetic's bootloader supports booting a LZMA compressed kernel but seems to fail if the uncompressed data is larger than a fixed buffer therefore it is safer to use a uimage-lzma-loader. When using OEM bootloader, the firmware image size cannot exceed the size of one OEM «Firmware_x» partition or Kernel + rootFS size.

Signed-off-by: Anton Yu. Ivanusev <ivanusevanton@yandex.ru>
Link: https://github.com/openwrt/openwrt/pull/17381
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>