mediatek: add support for Cudy WR3000 v1

mediatek: add support for Cudy WR3000 v1

Hardware
--------
MediaTek MT7981 WiSoC
256MB DDR3 RAM
16MB SPI-NOR (XMC XM25QH128C)
MediaTek MT7981 2x2 DBDC 802.11ax 2T2R (2.4 / 5)
UART: 115200 8N1 3.3V
      [LEDS] VCC-GND-RX-TX [ETH]
      Header is located below the heatsink

Case
----
Unscrew the 4 bottom screws. Remove the top of the case by inserting a
small screwdriver into the ventilation holes and lift the top cover.

This works best by beginning near the ETH-ports. The top is clipped on
the front near the LEDs with two plastic clips. The back has a single
clip in the middle. Start at one of the back edges.

MAC-Addresses
-------------
80:AF:CA:00:F9:C6 LAN
80:AF:CA:00:F9:C7 WAN
80:AF:CA:00:F9:C6 W2
82:AF:CA:30:F9:C6 W5

Installation
------------

1. Connect to the serial port as described in the "Hardware" section.

2. Power on the device. Keep pressing the "0" key to enter the U-Boot
   shell.

3. Download the OpenWrt initramfs image. Place it on an TFTP server
   connected to the Cudy LAN ports. Make sure the server is reachable at
   192.168.1.2. Rename the image to "cudy3000.bin"

4. Download and boot the OpenWrt initramfs image.

   $ tftpboot 0x46000000 cudy3000.bin; bootm 0x46000000

5. Transfer the OpenWrt sysupgrade image to the device using scp.
   Install with sysupgrade.

Note: Cudy does not yet provide a image for disabling their
signature-protection. This has happened in the past. Make sure to check
the wiki for a possible easier installation method.

Signed-off-by: David Bauer <mail@david-bauer.net>