mediatek: filogic: add H3C Magic NX30 Pro support

mediatek: filogic: add H3C Magic NX30 Pro support

Hardware specification:
  SoC: MediaTek MT7981B 2x A53
  Flash: W25N01GVZEIG 128MB
  RAM: NT5CB128M16JR-FL 256MB
  Ethernet: 4x 10/100/1000 Mbps
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset, WPS
  Power: DC 12V 1A

Flash instructions:
  1. PC run command: "telnet 192.168.124.1 99"
     Username: H3C, password is the web login
     password of the router.
  2. Download preloader.bin and bl31-uboot.fip
  3. PC run command: "python3 -m http.server 80"
  4. Download files in the telnet window:
     "wget http://192.168.124.xx/xxx.bin"
     Replace xx with your PC's IP and
     the preloader.bin and bl31-uboot.fip.
  5. Flushing openwrt's uboot:
     "mtd write xxx-preloader.bin BL2"
     "mtd write xxx-bl31-uboot.fip FIP"
  6. Connect to the router via the Lan port,
     set a static ip of your PC.
     (ip 192.168.1.254, gateway 192.168.1.1)
  7. Download initramfs image, reboot router,
     waiting for tftp recovery to complete.
  8. After openwrt boots up, perform sysupgrade.

Note:
  1. The u-boot-env partition on mtd is empty,
     OEM stores their env on ubi:u-boot-env.
  2. Back up all mtd partitions before flashing.

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>