mediatek/filogic: add Keenetic KN-3911 support

mediatek/filogic: add Keenetic KN-3911 support

Specification:
 - MT7981 CPU using 2.4GHz and 5GHz WiFi (both AX)
 - 512MB RAM
 - 128MB SPI NAND
 - 2 LEDs (green, orange)
 - 3 buttons (fn, reset, wps)
 - 2 2.5Gbit ethernet ports based on Airoha EN8811H phy

Serial Interface:
 - 3 Pins GND, RX, TX
 - Settings: 115200, 8N1

Notes:
 - The device supports dual boot mode

Flash instruction:
The only way to flash OpenWrt image is to use tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.1.2/24 and tftp server.
2. Rename "openwrt-mediatek-filogic-keenetic_kn-3911-squashfs-factory.bin"
   to "KN-3911_recovery.bin" and place it in tftp server directory.
3. Connect PC with ethernet port, press the reset button, power up
   the device and keep button pressed until status led start blinking.
4. Device will download file from server, write it to flash and reboot.

Signed-off-by: Maxim Anisimov <maxim.anisimov.ua@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16830
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>