ramips: Add support for Cudy M1300 v2

ramips: Add support for Cudy M1300 v2

The M1300 v2 is similar to the WR1300 series from Cudy. Differences:
- Only 1 LAN port
- No USB

Specifications:
- MT7621
- MT7603E (2.4G b/g/n) and MT7613BE (5G ac/n) wifi
- 128 MB RAM
- 16 MB flash

MAC Addresses:
- There is one on the label, e.g. xx:xx:xx:xx:xx:1C
- LAN (bottom connector) is the same as the label, e.g. xx:xx:xx:xx:xx:1C
- WAN (top connector) is label +2, e.g. xx:xx:xx:xx:xx:1E
- WLAN (2.4G) is the same as the label, e.g. xx:xx:xx:xx:xx:1C
- WLAN (5G) is the same as WAN, e.g. xx:xx:xx:xx:xx:1E

UART:
- is available via the pin holes on the board
- From inner to outer pin: TX, RX, GND, VCC
- Do NOT connect VCC
- Settings: 3.3V, 115200, 8N1

GPIO:
- There are two LEDs: Red (GPIO 3) and White (GPIO 4)
- There are two buttons: Reset (GPIO 8) and WPS (GPIO 10)

Migration to OpenWrt:
- Download the migration image from the Cudy website (it should be available as soon as OpenWrt officially supports the device)
- The migration image is also available here until a image is provided by Cudy: https://github.com/RolandoMagico/openwrt-build/releases/tag/M1300_Build_20240222
- File: openwrt-ramips-mt7621-cudy_m1300-v2-squashfs-flash-signed.bin
- Connect computer to LAN (bottom connector) and flash the migration image via OEM web interface
- In the migration image, LAN and WAN are swapped. Computer must be connected to the other port after flashing
- OpenWrt is now accessible via 192.168.1.1
- After flashing an up to date OpenWrt image, LAN and WAN settings are again the same as in the OEM firmware
- So use the other connector again

Revert back to OEM firmware:
- Set up a TFTP server on IP 192.168.1.88 and connect to the LAN port (lower port)
- Provide the Cudy firmware as recovery.bin in the TFTP server
- Press the reset button while powering on the device
- Recovery process is started now
- When recovery process is done, OEM firmware is accessible via 192.168.10.1 again

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

Signed-off-by: Roland Reinl <reinlroland+github@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18139
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>