ramips: add support for Hongdian H7920 v40

ramips: add support for Hongdian H7920 v40

This is an industrial 4G router equipped with OpenWrt SNAPSHOT OEM
customized version

WARNING: The original firmware device tree is modified from evb
boards, and the device tree name is evb board. This submitted device
tree is a modified version, which deletes the non-this-device parts
and adds GPIO watchdog.

Specification:
- SoC: MediaTek MT7628NN
- Flash: 16 MB
- RAM: 128 MB
- Power: DC 5V-36V 1.5A
- Ethernet: 1x WAN [slot not install], 1x LAN (10/100 Mbps)
- Wireless radio: 802.11n 2.4g-only [antenna not install]
- LED:
  System/Power (RUN): GPIO/37 active-low
  Modem: GPIO/3 active-low
  RF (Modem Signal): GPIO/2 active-low
- Button:
  WPS / RESET: GPIO/11 active-low
- UART: 1x UART on PCB - 115200 8N1
- Serial / COM: 1X RS232/RS485 on board (GPIO/6 hi:RS485 lo:RS232)
- GPIO Watchdog: GPIO/0 mode=toggle timeout=1s
- Modem: 1x Built-in modem on board (Power: GPIO/4 active-high)
- PCIe: 1x miniPCIe for modem [slot not install]
- SIM Slots: 1x SIM Slots

Issue:
- Factory partition not store mac address on original firmware

Flash instruction:
Using SSH/Telnet:
1. Connect the board to the computer via RJ45 Ethernet
2. Login 192.168.8.1 with root password "superzxmn" (SSH Port 22, Telnet Port 5188)
3. Download openwrt firmware on the computer.
4. Use scp or sftp put firmware to board /tmp
5. Use command "mtd -r write openwrt-ramips-mt76x8-hongdian_h7920-v40-squashfs-sysupgrade.bin firmware"
   to flash

Original Firmware Dump / More details:
https://blog.gov.cooking/archives/research-hongdian-h7920-v40-and-flash.html

Signed-off-by: Coia Prant <coiaprant@gmail.com>
Tested-by: Coia Prant <coiaprant@gmail.com>
(cherry picked from commit ebee9462273d3d30dbd29434e4f715ce39f80efc)
Link: https://github.com/openwrt/openwrt/pull/19716
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>