mediatek/filogic: add Keenetic KN-3811 support

mediatek/filogic: add Keenetic KN-3811 support

Specification:
 * Mediatek MT7981BA
 * 256 MB SPI-NAND
 * 512 MB DDR4 RAM
 * MT7976CN DBDC AX Wi-Fi
 * MediaTek MT7531AE (3x LAN Gigabit ports) + Internal Gbe Phy (1x WAN Gigabit port)
 * 4x LED (power, internet, fn, wifi)
 * 3x buttons (wps, fn, reset)
 * 1x USB 3.0 port

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

Notes:
 * The device supports dual boot mode
 * Fn led reassigned to wlan 2.4

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-3811-squashfs-factory.bin"
   to "KN-3811_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/17135
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>