ramips: add support for Wavlink WL-WN576A2

ramips: add support for Wavlink WL-WN576A2

This commit adds support for the Wavlink WL-WN576A2 wall-plug wireles
repeater / router. It is also sold under the name SilverCrest SWV 733 B1.

Device specs:

- CPU: MediaTek MT7628AN
- Flash: 8MB
- RAM: 64MB
- Bootloader: U-Boot
- Ethernet: 1x 10/100 Mbps
- 2.4 GHz: b/g/n SoC
- 5 GHz: a/n/ac MT7610EN
- Buttons: WPS, reset, sliding switch (ap/repeater)
- LEDs: 5x wifi status, 1x LAN/WAN, 1x WPS

Flashing:

U-Boot launches a TFTP client if WPS button is held during boot.

- Server IP: 192.168.10.100
- Firmware file name: firmware.bin

Device will reboot automatically. First boot takes about 90s.

Coelner (waenger@gmail.com) is the original author, but I have made some
fixes. He does not wish to sign off using his real name.

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