ramips: mt7621: add support for Cudy AP1300 Outdoor v1

ramips: mt7621: add support for Cudy AP1300 Outdoor v1

Hardware:
 - SoC: MediaTek MT7621DAT
 - Flash: 16 MiB XM25QH128C
 - RAM: 128 MiB
 - WLAN: 2.4 GHz (MT7603E, 11n), 5 GHz (MediaTek MT7613BEN, 11ac)
 - Ethernet: 1x10/100/1000 Mbps LAN
 - Buttons: 1 Reset button, 1 WPS button
 - LEDs: 5x Green
 - Serial Console: unpopulated header 115200 8n1 (silkscreen on PCB)
 - Power: POE 802.3af (37-57V DC)

MAC addresses:
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| LAN     | 80:af:ca:xx:xx:x0 | label     |
| WLAN 2g | 80:af:ca:xx:xx:x0 | label     |
| WLAN 5g | 82:af:ca:xx:xx:x1 | +1        |
+---------+-------------------+-----------+

Installation:
The factory firmware is locked: you can only work with Cudy signed firmware.
Download a intermediate firmware signed by Cudy here:
https://www.cudy.com/blogs/faq/openwrt-software-download

After that, login to the router (192.168.10.254, password "admin") and install the intermediate firmware.

If you can reach LuCI or SSH now on the intermediate firmware, just use the sysupgrade image with the 'Keep settings' option turned off.

Special thanks to Daniel de Kock for starting the porting work at #16265.

Signed-off-by: Luis Mita <luis@luismita.com>
Co-Authored-By: Daniel de Kock <daniel@riot.network>
Co-Authored-By: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://github.com/openwrt/openwrt/pull/20268
(cherry picked from commit 8fb644e9e8416df0d7051d8e35b57b06a030c05e)
Link: https://github.com/openwrt/openwrt/pull/20710
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>