mediatek: filogic: add Keenetic Sprinter (KN-3711) support

mediatek: filogic: add Keenetic Sprinter (KN-3711) support

Specification:
    SoC: Mediatek MT7981BA
    RAM: 512 MB DDR4 RAM
    Flash: Winbond W25N01GV 128 MB SPI-NAND (dual boot on OEM)
    WiFi: MT7976CN DBDC AX
    Switch: MediaTek MT7531AE (3x LAN Gigabit ports) + Internal Gbe Phy (1x WAN Gigabit port)
    GPIO: 4x LED (power, internet, fn, wifi), 2x buttons (wps, reset), 1x switch (mesh)

UART Interface:
    Pins: VCC, TX, RX, NC, GND
    Settings: 115200, 8N1

Flashing via TFTP:
    1. Connect your PC and router to the first LAN port, configure PC interface using IP 192.168.1.2, mask 255.255.255.0
    2. Save the factory image of the OpenWRT firmware, renamed to KN-3711_recovery.bin, on the TFTP server
    3. Hold the reset button and power up the device
    4. Keep the button pressed until the status LED starts blinking

Signed-off-by: Vladimir Epifantsev <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19633
Signed-off-by: Hauke Mehrtens <[email protected]>