ramips: add support for Hongdian H8922 v30

ramips: add support for Hongdian H8922 v30

This is an industrial 4G router equipped with OpenWrt 14.07 OEM
customized version

WARNING: The original firmware device tree is common to multiple
boards, and the device tree name is H9350. This submitted device
tree is a modified version, which deletes the non-this-device parts
and adds GPIO watchdog.

Specification:
- SoC: MediaTek MT7620A
- Flash: 16 MB
- RAM: 128 MB
- Power: DC 5V-36V 1.5A
- Ethernet: 1x WAN, 4x LAN (10/100 Mbps)
- Wireless radio: 802.11n 2.4g-only
- LED:
  System/Power (RUN): GPIO/26 active-low
  Ethernet: 1x WAN, 4x LAN
  Modem 1: GPIO/66 active-low
  RF 1 (Modem 1 Signal): GPIO/67 active-low
  Modem 2: GPIO 71 active-low
  RF 2 (Modem 2 Signal): GPIO/24 active-low
  WLAN: GPIO/72 active-low
  WPS: GPIO/12 active-low
- Button:
  WPS / RESET: GPIO/34 active-low
- UART: 1x UART on PCB - 115200 8N1
- GPIO Watchdog: GPIO/62 mode=toggle timeout=1s
- PCIe: 2x miniPCIe for modem
- SIM Slots: 2x SIM Slots

Issue:
- No factory partition, eeprom is located
 at /lib/firmware/mt7620a.eeprom

Flash instruction:
Using UART:
1. Configure PC with a static IP address and setup an TFTP server.
2. Put rootfs into the tftp directory.
3. Connect the UART line as described on the PCB.
4. Power up the device and press Ctrl+C to break auto boot.
5. Use `system 6` command and follow the instruction to set device
   and tftp server IP address and input the rootfs file name.
   U-boot will then load the rootfs and write it into
   the flash.
6. Use `system 1` command and follow the instruction to set device
   and tftp server IP address and input the firmware file name.
   U-boot will then load the firmware once.
7. Login to LuCI and use LuCI upgrade firmware.

Original Firmware Dump / More details:
https://blog.gov.cooking/archives/research-hongdian-h8922-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/17472
Link: https://github.com/openwrt/openwrt/pull/18221
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 6a1bdcf545eec8f9ed5130e7f5794fca6ee54cb3)