ramips: add support for WAVLINK WL-WN577A2

ramips: add support for WAVLINK WL-WN577A2

This commit adds support for the Wavlink WL-WN577A2 (black case) dual-band
wall-plug wireless router. In Germany this device is sold under the brand
name Maginon WL-755 (white case):

Device specifications:

- CPU: MediaTek MT7628AN (580MHz)
- Flash: 8MB
- RAM: 64MB
- Bootloader: U-Boot
- Ethernet: 2x 10/100 Mbps (Ralink RT3050)
- 2.4 GHz: 802.11b/g/n SoC
- 5 GHz: 802.11a/n/ac MT7610E
- Antennas: internal
- 4 green LEDs: 1 programmable (WPS) + LAN, WAN, POWER
- Buttons: Reset, WPS
- Small sliding power switch

Flashing instructions (U-boot):

- Configure a TFTP server on your PC/Laptop and set its IP
  to 192.168.10.100

- Rename the OpenWrt image to firmware.bin and place it in the
  root folder of the TFTP server

- Power off (using the small sliding power switch on the left
  side) the device and connect an ethernet cable from its LAN
  or WAN port to your PC/Laptop

- Press the WPS button (and keep it pressed)

- Power on the device (using the small power switch)

- After a few seconds, when the WAN/LAN LED stops blinking
  very fast, release the WPS button

- Flashing OpenWrt takes less than a minute, system will
  reboot automatically

- After reboot the WPS LED will indicate the current OpenWrt
  running status

Signed-off-by: Lars Wessels <software@bytebox.org>
[removed unused labels - fix whitespace errors - wrap commit message]
Signed-off-by: David Bauer <mail@david-bauer.net>