mediatek: filogic: add Teltonika RUTC50 support

mediatek: filogic: add Teltonika RUTC50 support

Specification:
- MediaTek MT7981A SoC
- 512 MB RAM
- 16MB SPI NOR Flash
- 512MB NAND (split in half for firmware fallback)
- 5x 10/100/1000 Mbps Ethernet, with passive PoE support on LAN1
- WLAN      : MediaTek dual-band WiFi 6
  - 2.4 GHz : b/g/n/ax, MIMO 2x2
  - 5 GHz   : a/n/ac/ax, MIMO 2x2
- Quectel RG520N-EB 5G R16 modem
- 2.0 USB Type-A HOST port
- 1x Digital input
- 1x Digital output
- 2x SIM slot (can be swapped via AT commands)

GPIO:
- 1 button (Reset)
- 13 LEDs (power, 4x WAN status, Wifi 2G, Wifi 5G, 3G, 4G, 5G, RSSI 1,2,3)
- 3 Modem control (power button, reset, status)
- 1 USB reset
- 1 Digital input
- 1 Digital output

Flashing via OEM WebUI:
1. Download the firmware image *-squashfs-factory.bin
2. Upload firmware image via OEM WebUI firmware update, do not keep settings

To revert back to OEM firmware:
https://wiki.teltonika-networks.com/view/Bootloader_menu

Use "ModemManager" to establish mobile data connection.

Signed-off-by: Simonas Tamošaitis <simsasss@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19401
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>