ramips: add support for Wavlink WL-WN530HG4

ramips: add support for Wavlink WL-WN530HG4

Same hardware as Phicomm K2G but different flash layout.

Specification:
- SoC: MediaTek MT7620A
- Flash: 8 MB
- RAM: 64 MB
- Ethernet: 4 FE ports and 1 GE port (RTL8211F on port 5)
- Wireless radio: MT7620 for 2.4G and MT7612E for 5G, both equipped
  with external PA.
- UART: 1 x UART on PCB - 57600 8N1

Flash instruction:
To avoid requiring UART for TFTP a dual flash procedure is suggested
to install the squashfs image:
1. Rename openwrt-ramips-mt7620-wavlink_wl-wn530hg4-initramfs-kernel.bin
   to WN530HG4-WAVLINK.
2. Flash this file with the factory web interface.
3. With OpenWRT now running use standard sysupgrade to install the
   squashfs image.

Signed-off-by: Nuno Goncalves <nunojpg@gmail.com>
[remove dts-v1, remove model from LED labels, wrap commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>