mediatek: add support for Imou HX21

mediatek: add support for Imou HX21

This board is also as known as LC-HX3001

Hardware specification:
  SoC: MediaTek MT7981B 2x A53
  Flash: Foresee F35SQA001G 128 MB SPI-NAND
  RAM: 256MB DDR3
  Ethernet: 4x 10/100/1000 Mbps
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset, Mesh
  Power: DC 12V 1A

Gain SSH access:
Method 1:
1. Connect UART Adapter to the router
2. UART pins are enabled, set the root password with passwd command
   start the dropbear instance on port 22

Method 2:
1. Login into web interface, and restore the router with SSH enabled configuration.
2. After reboot Default Web interface password will be 12345678
   SSH password is empty

Flash instructions:
1. Connect to HX21, backup everything, especially 'Factory' part.
3. Write new FIP:
   mtd write openwrt-mediatek-filogic-hx_21-bl31-uboot.fip FIP
4. Set static IP on your PC:
   IP 192.168.1.254/24, GW 192.168.1.1
5. Serve OpenWrt initramfs image using TFTP server.
6. Wait for TFTP recovery to complete.
7. After OpenWrt has booted, perform sysupgrade.
8. To write new BL2
   Install mtd-rw
   opkg update && opkg install kmod-mtd-rw
   insmod mtd-rw i_want_a_brick=1
   mtd write openwrt-mediatek-filogic-imou_hx21-preloader.bin bl2

Signed-off-by: Jahidul Islam <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20753
Signed-off-by: Hauke Mehrtens <[email protected]>