ramips: add support for Keenetic KN-1613

ramips: add support for Keenetic KN-1613

Keenetic KN-1613 is a 2.4/5 Ghz band 11ac (Wi-Fi 5) router, based on MT7628AN.

Specification:
- System-On-Chip: MT7628AN
- CPU/Speed: 580 MHz
- Flash-Chip: Winbond w25q256
- Flash size: 32768 KiB
- RAM: 128 MiB
- 4x 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
- Wireless No2 (2T2R): MT7613BE 5 GHz 802.11ac
- 4x LED, 2x button, 1x mode switch

Notes:
- The device supports dual boot mode
- The firmware partitions were concatinated into one
- The FN button led indicator has been reassigned as the 2.4GHz
  wifi indicator.

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-mt76x8-keenetic_kn-1613-squashfs-factory.bin"
   to "KN-1613_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>