ramips: add support for Wavlink WL-WN578A2

ramips: add support for Wavlink WL-WN578A2

This commit adds support for the Wavlink WL-WN578A2 dual-band wall-plug
wireless router. This device is also sold under the name SilverCrest
SWV 733 A2.

Device Specifications:

- CPU: MediaTek MT7628AN (580MHz)
- Flash: 8MB
- RAM: 64MB
- Bootloader: U-Boot
- Ethernet: 2x 10/100 Mbps
- 2.4 GHz: 802.11b/g/n SoC
- 5 GHz: 802.11a/n/ac MT7610E
- Antennas: internal
- 4 green LEDs: WPS/Power, LAN, WAN, wifi-low, wifi-med, wifi-high
- Buttons: Reset, WPS
- Sliding mode switch: AP, repeater, client
- Small sliding power switch

Flashing instructions:

U-Boot launches TFTP client if WPS button is pressed during power-on.
Configure as follows:

- Server IP: 192.168.10.100
- Filename (rename sysupgrade file to this): firmware.bin

Flashing should not take more than a minute, device will reboot
automatically.

Signed-off-by: Thomas Aldrian <dev.aldrian@gmail.com>