mediatek: add support for TP-Link TL-XTR8488

mediatek: add support for TP-Link TL-XTR8488

Hardware specification:
  SoC: MediaTek MT7986A 4x A53
  Flash: ESMT F50L1G41LB 128MB
  RAM: MT40A512M16TB-062ER 1GB
  Ethernet: 2x 2.5G, 4x 1G Lan
  WiFi1: MT7976GN 2.4GHz 4T4R
  WiFi2: MT7976AN 5.2GHz 4T4R
  WiFi3: MT7915AN 5.8GHz 4T4R
  Button: Reset, WPS, Turbo
  USB: 1 x USB 3.0
  Power: DC 12V 5A

Flash instructions:
  1. Execute the following operation to open nc shell:
     https://openwrt.org/inbox/toh/tp-link/xdr-6086#rooting
  2. Replace the stock bootloader to OpenWrt's:
     dd bs=131072 conv=sync of=/dev/mtdblock9 if=/tmp/xxx-preloader.bin
     dd bs=131072 conv=sync of=/dev/mtdblock9 seek=28 if=/tmp/xxx-bl31-uboot.fip
  3. Connect to your PC via the Gigabit port of the router,
     set a static ip on the ethernet interface of your PC.
  4. Download the initramfs image, and restart the router,
     waiting for tftp recovery to complete.
  5. After openwrt boots up, perform sysupgrade.

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Link: https://github.com/openwrt/openwrt/pull/15930
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>