ramips: mt76x8: add support for Keenetic KN-3211

ramips: mt76x8: add support for Keenetic KN-3211

Keenetic KN-3211 is a 2.4 Ghz band 11n (Wi-Fi 4) Wi-Fi repeater, based on MT7628AN.

Specification:
- System-On-Chip: MT7628AN
- CPU/Speed: 575 MHz
- Flash-Chip: Winbond w25q256
- Flash size: 32768 KiB
- RAM: 64 MiB
- 1x 10/100 Mbps Ethernet
- 2x external, non-detachable antennas
- UART (J1) header on PCB (115200 8n1)
- Wireless No1 (2T2R): SoC Built-in 2.4 GHz 802.11bgn
- 3x LED, 1x button, 1x mode switch

Notes:
- The device supports dual boot mode
- The firmware partitions were concatenated into one
- The status button has been reassigned as the WPS button.

Flash instruction:
This device doesn't support sysupgrade, so 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-ramips-mt76x8-keenetic_kn-3211-squashfs-factory.bin"
   to "KN-3211_recovery.bin" and place it in tftp server directory.
3. Connect PC with the ethernet port, press the reset button, power up
   the router and keep button pressed until power led starts blinking.
4. Router will download file from server, write it to flash and reboot.

Signed-off-by: Ivan Davydov <lotigara@lotigara.ru>
Link: https://github.com/openwrt/openwrt/pull/17080
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>