ramips: add support for 7Links WLR-1240

ramips: add support for 7Links WLR-1240

The WLR-1240 (ZX-5434) is an AC1200 Wave 2 outdoor repeater
with omnidirectional antennas for wall or pole mounting.
The device is manufactured by Todaair and meant to be used with
a tuya-based app, there is no webinterface for configuration.

Specifications:
- MT7628AN, 8 MiB SPI NOR flash, 64 MiB RAM, 2x2 802.11n
- MT7613 2x2 802.11ac Wave 2
- 802.3af PoE or 12V 1A 5.5x2.1 power supply (included)
- top RGB LED ring

TFTP installation:
- rename sysupgrade to `firmware_auto.bin`
- provide at 192.168.1.10 during boot

HTTP installation:
- keep reset button pressed for 5 seconds during power on (light blue
  LED flashes slowly, then quickly to confirm, then remains steady on)
- recovery web interface is at 192.168.1.1, upload sysupgrade

Opening the device
- use suction cup to remove top cap within LED ring
- two screws are located in holes underneath silicone sealant
- two further screws are located at the bottom

initramfs boot
- open device, connect serial console (pins are labelled)
- keep pressing `4` during second tftp attempt to enter uboot shell
- run `tftpboot 82000000` to avoid memory overlap, then `bootm`

Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>