ramips: mt76x8: add Teltonika RUT976 support

ramips: mt76x8: add Teltonika RUT976 support

This commit adds support for the Teltonika RUT976, part
of the RUTE series, which is very similar to the RUT956.
The RUT976 now has a 5G modem and the flash size has doubled.

Specification:
- MediaTek MT7628AN SoC
- 128 MB RAM
- 32MB Flash
- 4x 10/100 Mbps Ethernet, with passive PoE support on LAN1
- MediaTek MT7628AN 2.4 GHz 802.11n WiFi
- Quectel RG255C 5G modem
- GNSS
- RS232, DB9 connector, Cypress ACM via USB, /dev/ttyACM0
- RS485 /dev/ttyS1
- microSD card slot
- 2.0 USB Type-A HOST port
- analog 0-24V input (MCP3221)
- Relay
- 2x Digital input
- 2x 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)
 - 2 Digital input
 - 2 Digital output
 - 1 Isolated input
 - 1 RS485 tx enable
 - 1 RS485 rx enable
 - 1 Relay
 - 1 Data Carrier Detect (DCD)

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:
Create proto qmi interface.

Signed-off-by: Til Kaiser <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20933
Signed-off-by: Hauke Mehrtens <[email protected]>