mediatek: filogic: add support for TP-LINK RE6000XD

mediatek: filogic: add support for TP-LINK RE6000XD

This commit adds support for TP-LINK RE6000XD.
The device is quite similar to the Mercusys MR90X V1,
except only 3 LAN ports and more LEDs.
So thanks to csharper2005 for doing all the groundwork.

Device specification
--------------------
SoC Type:   MediaTek MT7986BLA, Cortex-A53, 64-bit
RAM:        MediaTek MT7986BLA (512MB)
Flash:      SPI NAND GigaDevice (128 MB)
Ethernet:   MediaTek MT7531AE + 2.5GbE MaxLinear GPY211C0VC (SLNW8)
Ethernet:   1x2.5Gbe (LAN3 2.5Gbps), 2xGbE (LAN 1Gbps, LAN1,
            LAN2)
WLAN 2g:    MediaTek MT7975N, b/g/n/ax, MIMO 4x4
WLAN 5g:    MediaTek MT7975P(N), a/n/ac/ax, MIMO 4x4
LEDs:       8 LEDs, 1 status blue, 2x WIFI blue, 2x signal
	    blue/red, 3 LAN blue gpio-controlled
Button:     2 (Reset, WPS)
USB ports:  No
Power:      12 VDC, 2 A
Connector:  Barrel
Bootloader: Main U-Boot - U-Boot 2022.01-rc4. Additionally, ubi0
            partition contain "seconduboot" (also U-Boot 2022.01-rc4)

Serial console (UART), unpopulated
---------------------
                            V
+-------+-------+-------+-------+
| +3.3V |  GND  |  TX   |  RX   |
+---+---+-------+-------+-------+
    |
    +--- Don't connect

Disassemble: rm the 2 screws at the bottom and the one at the backside.
            un-clip the case starting at the edge above the LEDs.

Installation (UART)
-------------------
1. Place OpenWrt initramfs image on tftp server with IP 192.168.1.2
2. Attach UART, switch on the router and interrupt the boot process by
   pressing 'Ctrl-C'
3. Load and run OpenWrt initramfs image:
      tftpboot openwrt-mediatek-filogic-tplink_re6000xd-initramfs-kernel.bin bootm
4. Run 'sysupgrade -n' with the sysupgrade OpenWrt image

Notice: while I was successfull at activating ssh (as described
        here:
        https://www.lisenet.com/2023/gaining-ssh-access-to-tp-link-re200-wi-fi-range-extender/)

        Unfortunately I haven't found the correct root password.
        Looks like they are using a static password
        (md5crypt, salt + 21 characters) that is not the web
        interface admin password.

        The TP-LINK RE900XD looks like the very same device,
        according to the pictures and the firmware.
        But I haven't checked if the OpenWrt firmware works as well
        on that device.

        The second ubi partition (ubi1) is empty and there is no known
        dual-partition mechanism, neither in u-boot nor in the stock firmware.

Signed-off-by: Dirk Buchwalder <buchwalder@posteo.de>