ramips: add support for Keenetic KN-3510

ramips: add support for Keenetic KN-3510

Keenetic KN-3510 is a 2.4/5 Ghz band 11ax access point

Specification:
- System-On-Chip: MT7621AT
- CPU/Speed: 880 MHz
- Flash-Chip: Macronix MX30LF1G28AD-TI
- Flash size: 128 MiB
- RAM: 256 MiB
- 2x 10/100/1000 Mbps Ethernet
- PoE, 802.3af/at
- 4x internal antennas
- UART (J1) header on PCB (115200 8n1)
- WiFi: MT7915 2x2 2.4G 573.5Mbps + 2x2 5G 1201Mbps
- 2x LED, 2x button, 1x mode switch

Notes:
- The device supports dual boot mode
- The firmware partitions were concatinated into one

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-ramips-mt7621-keenetic_kn-3510-squashfs-factory.bin"
   to "KN-3510_recovery.bin" and place it in tftp server directory.
3. Connect PC with one of LAN ports, press the reset button, power up
   the router and keep button pressed until power led start blinking.
4. Router 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/15744
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>