mediatek: filogic: add TP-Link TL-XDR6086 and TL-XDR6088

mediatek: filogic: add TP-Link TL-XDR6086 and TL-XDR6088

Add support for TP-Link TL-XDR6086 and TL-XDR6088 WiFi 6 routers.

Specs:
 * SoC: MediaTek MT7986A
 * RAM: DDR3 512MiB (ESMT M15T4G16256A)
 * Flash: SPI-NAND 128 MiB (ESMT F50L1G41LB)
 * WLAN:
    2.4G: MediaTek MT7976GN
    5G: MediaTek MT7976AN
 * Ethernet:
    GMAC0 connected to MediaTek MT7531A
     = 1x gigE port + 1x Realtek RTL8221B on TL-XDR6086
     = 4x gigE port + 1x Realtek RTL8221B on TL-XDR6088
    GMAC1 connected to Realtek RTL8221B
 * USB: 1x USB 3.0
 * LEDs:
   * Status: red/green
   * Turbo: green
 * Buttons: Reset, WPS/Turbo (?)
 * Power : 12 VDC, 4A

As the bootloader of the device is locked down, installation is currently
only possible by using a root-shell exploit in the stock firmware and
subsequently writing bl2 and fip images. Fortunately the stock firmware
offers raw access to the flash chip as a whole via /dev/mtd9...

dd bs=131072 conv=sync of=/dev/mtdblock9 if=/tmp/openwrt-mediatek-filogic-tplink_tl-xdr6086-preloader.bin
dd bs=131072 conv=sync of=/dev/mtdblock9 seek=28 if=/tmp/openwrt-mediatek-filogic-tplink_tl-xdr6086-bl31-uboot.fip
(replace xdr6086 with xdr6088 depending on the actual device you got)

Based on initial steps done by @soxrok2212 and device tree improvements
and support for TL-XDR6088 done by @aiamadeus.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>