ramips: mt76x8: add Teltonika RUT951/RUT901 support

ramips: mt76x8: add Teltonika RUT951/RUT901 support

Specification:
- MediaTek MT7628AN SoC
- 128 MB of RAM EtronTech EM68C16CWQG-25IH
- 16MB of Flash Winbond W25Q128 SPI
- 4x 10/100 Mbps Ethernet, with passive PoE support on LAN1
- MediaTek MT7628AN 2.4 GHz 802.11n WiFi
- Quectel EC25-EUX 4G cat 4 modem (RUT951) or Quectel EC200A-EU 4G cat 4 modem (RUT901)
- 1x Digital input
- 1x Digital output
- 2x SIM slot (can be swapped via GPIO)
- eSIM (depends on hardware configuration, can be swapped via GPIO)

GPIO:
 - 1 button (Reset)
 - 7 LEDs (power, status green/red, RSSI 1,2,3,4,5)
 - 5 Modem control (power button, reset, status, SIM select, eSIM select)
 - 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

Mobile data connection:
Quectel EC200A-EU send command to modem:
    echo -ne 'AT+QNETDEVCTL=3,1\r\n' > /dev/ttyUSB2
    Create DHCP interface with usb0 device.
Quectel EC25-EUX create proto qmi interface.

Signed-off-by: Simonas Tamošaitis <simsasss@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18788
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>