ramips: Add support for Wavlink WL-WN575A3

This commit adds support for the Wavlink WL-WN575A3, a dual-band wall-plug
wireless router with the following specifications:

 - CPU: MediaTek MT7628AN 580MHz
 - Flash: 8MB
 - RAM: 64MB
 - Ethernet: 2x 10/100 Mbps (switched)
 - 2.4 GHz: 802.11b/g/n SoC, MIMO 2x2, 20 dBm
 - 5 GHz: 802.11a/n/ac MT7612E, MIMO 2x2, 20 dBm
 - Antennas: 4x external (2 per radio), non-detachable
 - LEDs: 4 programmable + LAN, WAN, POWER
 - Buttons: reset, WPS

Flashing instructions:

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

 - Client (WL-WN575A3) IP: 192.168.10.101
 - Server IP: 192.168.10.100
 - Filename: firmware.bin

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>