ramips: add support for Hongdian H8850 v20

ramips: add support for Hongdian H8850 v20

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 MT7628DAN
- Flash: 16 MB
- RAM: 64 MB
- Power: DC 5V-36V 1.5A
- Ethernet: 1x WAN/LAN, 3x LAN (10/100 Mbps)
- Wireless radio: 802.11n 2.4g-only
- LED:
  System/Power (PWR): Always Off
  Modem (NET): GPIO/3 active-low
  LAN: Always On
  RF (Modem Signal): GPIO/2 active-low
  WIFI: GPIO/44 active-low
- Button:
  WPS / RESET: GPIO/11 active-low
- UART: 1x UART on PCB - 115200 8N1
- GPIO Watchdog: GPIO/0 mode=toggle timeout=1s
- Modem: 1x Built-in modem on board (Power: GPIO/4 active-high)
- SIM Slots: 1x SIM Slots

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

Flash instruction:
Using TTL:
1. Connect the board to the computer via TTL.
2. Enter original firmware failsafe mode.
3. Use wget download firmware to board /tmp
4. Use command "mtd -r write openwrt-ramips-mt76x8-hongdian_h8850-v20-squashfs-sysupgrade.bin firmware"
   to flash

Original Firmware Dump / More details:
https://blog.gov.cooking/archives/research-hongdian-h8850-v20-and-flash.html

Signed-off-by: Coia Prant <coiaprant@gmail.com>
Tested-by: Coia Prant <coiaprant@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20259
(cherry picked from commit 23a070dfb1fd3ed7659fad4934a6a38d8693326f)
Link: https://github.com/openwrt/openwrt/pull/21007
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>