ramips: add support for Wavlink WL-WN570HA1

ramips: add support for Wavlink WL-WN570HA1

This commit adds support for the Wavlink WL-WN570HA1, a dual-band PoE
wireless router with the following specifications:

 - CPU: MediaTek MT7688AN 580MHz
 - Flash: 8MB
 - RAM: 64MB
 - Ethernet: 1x 10/100Mbps
 - 2.4 GHz: 802.11b/g/n SoC, 1T1R, 27 dBm
 - 5 GHz: 802.11a/n/ac MT7610E, 1T1R, 25 dBm
 - Antennas: 2x external (1 per radio), detachable
 - LEDs: 3 programmable + Wi-Fi, WAN/LAN, Power
 - Buttons: Reset

Flashing instructions:

Factory U-boot launches a TFTP client if reset button is pressed during power-on.
Rename the sysupgrade file and configure TFTP as follows:

 - Client (WL-WN570HA1) IP: 192.168.10.101
 - Server IP: 192.168.10.100
 - Filename: firmware.bin

Signed-off-by: Thomas Vincent-Cross <me@tvc.id.au>
[use generic board detection, add firmware partition compatible]
Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from a160f50d4f240019fafe95e80701ebb871c788fe)