ramips: mt76x8: add support for Cudy LT500 Outdoor v1

ramips: mt76x8: add support for Cudy LT500 Outdoor v1

Hardware:
 - SoC: MediaTek MT7628AN
 - Flash: 16 MiB SPI NOR
 - RAM: 128 MiB DDR2
 - WLAN: 2.4 GHz (MT7628AN, 11n), 5 GHz (MediaTek MT7615E, 11ac)
 - Ethernet: 1x10/100 Mbps LAN
 - Buttons: 1 Reset button
 - LEDs: 8x Green
 - LTE: internal USB-connected modem Quectel EC200A
 - Serial Console: unpopulated header 115200 8n1

MAC addresses:
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| LAN     | 80:af:ca:xx:xx:x0 | label     |
| WLAN 2g | 80:af:ca:xx:xx:x0 | label     |
| WLAN 5g | 80:af:ca:xx:xx:x2 | +2        |
+---------+-------------------+-----------+

Migration to OpenWrt:
- Download the RSA signed intermediate firmware:
  `openwrt-ramips-mt76x8-cudy_lt500-outdoor-v1-squashfs-flash.bin`
- Connect computer to LAN and flash the intermediate firmware via OEM web interface
- OpenWrt is now accessible via 192.168.1.1

Revert back to OEM firmware:
- Press the reset button while powering on the device
- Connect the LAN port to the PC
- Open 192.168.1.1 in a browser and use the wizard to upload and flash OEM firmware image
- When recovery process is done, OEM firmware is accessible via 192.168.10.1 again

Signed-off-by: rodriguezst <git@rodriguezst.es>
Link: https://github.com/openwrt/openwrt/pull/23752
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>