mediatek: add Xiaomi Redmi Router AX6000 support

mediatek: add Xiaomi Redmi Router AX6000 support

Hardware specification:
  SoC: MediaTek MT7986A 4x A53
  Flash: ESMT F50L1G41LB 128 MB
  RAM: K4A4G165WF-BCWE 512 MB
  Ethernet: 4x 10/100/1000 Mbps
  WiFi1: MT7976GN 2.4GHz ax 4x4
  WiFi2: MT7976AN 5GHz ax 4x4
  Button: Mesh, Reset

Flash instructions:
  1. Gain ssh and serial port access, see the link below:
     https://openwrt.org/toh/xiaomi/redmi_ax6000#installation
  2. Use ssh or serial port to log in to the router, and
     execute the following command:
     nvram set boot_wait=on
     nvram set flag_boot_rootfs=0
     nvram set flag_boot_success=1
     nvram set flag_last_success=1
     nvram set flag_try_sys1_failed=8
     nvram set flag_try_sys2_failed=8
     nvram commit
  3. Set a static ip on the ethernet interface of your computer
     (e.g. default: ip 192.168.31.100, gateway 192.168.31.1)
  4. Download the initramfs image, rename it to initramfs.bin,
     and host it with the tftp server.
  5. Interrupt U-Boot and run these commands:
     setenv mtdparts nmbm0:1024k(bl2),256k(Nvram),256k(Bdata),2048k(factory),2048k(fip),256k(crash),256k(crash_log),112640k(ubi)
     saveenv
     tftpboot initramfs.bin
     bootm
  6. After openwrt boots up, use scp or luci web
     to upload sysupgrade.bin to upgrade.

Revert to stock firmware:
  Restore mtdparts back to default, then use the
  vendor's recovery tool (Windows only).

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