mediatek: add support for TP-Link TL-XDR4288/608x

mediatek: add support for TP-Link TL-XDR4288/608x

Hardware specification:
  SoC: MediaTek MT7986A 4x A53
  Flash: ESMT F50L1G41LB 128MB
  RAM: ESMT M15T4G16256A 512MB
  Ethernet (Max Speed):
    XDR4288: 1x 2.5G Wan, 1x 2.5G Lan, 4x 1G Lan
    XDR6086: 1x 2.5G Wan, 1x 2.5G Lan, 1x 1G Lan
    XDR6088: 1x 2.5G Wan, 1x 2.5G Lan, 4x 1G Lan
  WiFi:
    XDR4288: MT7976DAN (2.4G 2T2R, 5G 3T3R)
    XDR6086/XDR6088:
      WiFi1: MT7976GN 2.4GHz 4T4R
      WiFi2: MT7976AN 5GHz 4T4R
  Button: Reset, WPS, Turbo
  USB: 1 x USB 3.0
  Power: DC 12V 4A

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.
     (ip 192.168.1.254, gateway 192.168.1.1)
  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>
[Add uboot build, fit and sysupgrade support, fix RealTek PHYs]
Signed-off-by: Daniel Golle <daniel@makrotopia.org>