ramips: mt76x8: add support for MovingComm C120EV

ramips: mt76x8: add support for MovingComm C120EV

Add support for the MovingComm C120EV 4G LTE router based on MT7628AN.

Hardware specification:
- SoC: MediaTek MT7628AN (580MHz)
- RAM: 64 MiB DDR2
- Flash: 16 MiB SPI NOR
- WiFi: 2.4 GHz (internal) + 5 GHz (MT7663 via PCIe)
- Ethernet: 4x 10/100Mbps
- USB: 1x USB 2.0 (internal only)
- Modem: Thinkwill ML7820+ (manual configuration required)
- UART: 57600 8N1
- Power: 12V DC, 1A

Flash instructions:
The stock firmware is OpenWrt-based (Chaos Calmer 15.05.1).
Flash the OpenWrt sysupgrade image via vendor Web UI at 192.168.99.1
(admin/admin), section System - Firmware Upgrade.

Recovery (requires UART access):
- UART: Connect to ttyS0 @ 57600, press 4 during boot delay (5 seconds)
- TFTP: Server 10.10.10.3, client 10.10.10.123, load image to 0x80000000

MAC address layout:
- LAN: factory 0x04
- WAN: factory 0x28 (02_network)
- 2.4GHz: factory 0x2e
- 5GHz: factory 0x8004 (PCIe EEPROM)

Signed-off-by: librarat876 <bloproton92@proton.me>
Link: https://github.com/openwrt/openwrt/pull/23273
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>