ramips: mt7621: add Teltonika RUTM30/RUTM31 support

ramips: mt7621: add Teltonika RUTM30/RUTM31 support

Specifications:
- MediaTek MT7621AT SoC
- 256 MB RAM
- 16MB SPI NOR Flash
- 256MB NAND (split in half for firmware fallback)
- 2x 10/100/1000 Mbps Ethernet, with passive PoE support on LAN
- WLAN      : MediaTek dual-band WiFi 5
  - 2.4 GHz : b/g/n, MIMO 2x2
  - 5 GHz   : n/ac, MIMO 2x2
- Quectel RG520N-EB 5G R16 modem (RUTM30) or RG500U-EB 5G (RUTM31)
- 1x Digital input
- 1x Digital output
- 2x SIM slot (can be swapped via AT commands)
- eSIM
- TPM

GPIO:
- 1 button (Reset)
- 3 LEDs (power, 2 RGB)
- 3 Modem control (power button, reset, eSIM switch)
- 1 Digital input
- 1 Digital output
- 1 TPM enable

Installation
------------
Notice: update OEM firmware to 7.19 or later, earlier versions will
fail to flash openwrt factory firmware.

1. Check from which partition the device is currently running from

  $ cat /proc/bootconfig/chosen

  In case this output reads rutos-b, install a software update from
  Teltonika first. After upgrade completion, check this file now reads
  rutos-a before continuing.

2. Download the *-squashfs-factory.bin firmware image
3. Flash firmware image via WEB interface

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

Mobile connection:
- RG520N-EB:
    Use "ModemManager" to establish mobile data connection.
- RG500U-EB:
    echo -ne 'AT+QNETDEVCTL=1,3,1\r\n' > /dev/ttyUSB2
    Create DHCP interface with usb0 device.

Signed-off-by: Simonas Tamošaitis <simsasss@gmail.com>