ramips: add support for Renkforce WS-WN530HP3-A

This adds support for the Renkforce WS-WN530HP3-A ceiling-
mountable Wireless Access Point, which is powered over PoE.
Hardware:
	- SoC: Mediatek MT7621DAT
	- RAM: 128MiB on SoC
	- Flash: 16MiB GigaDevice GD25Q128C
	- 2.4Ghz Wifi: Mediatek MT603EN
	- 5GHz Wifi: MT613BEN
	- Ethernet:
	  - 1x 1GBit WAN port, passive PoE capable
	  - 2x 1GBit LAN ports
	LEDs: 1x Bi-Color LED (red/blue)
	Buttons: 1x Reset Button, 1x Power Button

Installation:
	Power on the access point and immedately press the reset
	button for 10 seconds. Connect web-browser to 192.168.10.1
	and upload sysupgrade image. Flash uploaded image and wait
	about 2 minutes for reboot.

Signed-off-by: Birger Koblitz <mail@birger-koblitz.de>
Signed-off-by: Petr Štetiar <ynezz@true.cz> [fixed SoB]