ramips: mt76x8: add support for Cudy WR300 v1

ramips: mt76x8: add support for Cudy WR300 v1

Hardware:
 - SoC: MediaTek MT7628AN (MIPS 580MHz)
 - Flash: 8 MiB NOR
 - RAM: 64 MiB DDR2
 - WLAN: 2.4 GHz (MT7628)
 - Ethernet: 1x 10/100 Mbps WAN, 3x 10/100 LAN (MT7628)
 - Buttons: 1x Reset, 1x wps
 - LEDs: Front: 1x Red, 1x White
 - Serial console: unpopulated header, 115200 8n1
 - Power: 12v barrel

MAC addresses:
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| LAN/WAN | d4:0d:ab:xx:xx:x0 | label     |
| WLAN 2g | d4:0d:ab:xx:xx:x1 | label+1   |
+---------+-------------------+-----------+

Migration to OpenWrt:
- Download the RSA-signed intermediate firmware from the Cudy website: `openwrt-ramips-mt76x8-cudy_wr300-squashfs-flash.bin`
- Connect the computer to the LAN and flash the intermediate firmware via the OEM web interface
- OpenWrt is now accessible via 192.168.1.1

Revert to OEM firmware:
- Set up a TFTP server on IP 192.168.1.88 and connect to the WAN port (upper port)
- Provide the Cudy firmware as `recovery.bin` in the TFTP server
- Power on the device and hold the reset button immediately after the first LED blink
- The recovery process will start
- When the recovery process is done, OEM firmware is accessible via 192.168.10.1 again

General information:
- No possibility to load an initramfs image via U-Boot because there is no option to interrupt U-Boot

Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23426
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>