mediatek: add support for Zyxel WX5600-T0

mediatek: add support for Zyxel WX5600-T0

This access point is a ‘friend’ of the T56 supplied by Odido but with DDR3 RAM and with two Ethernet 2.5 (GPY211)

The flash procedure is similar to other Zyxel T56/EX5600/EX5601

If you need backup please use the T56 guide
Please refer to https://openwrt.org/inbox/toh/zyxel/wx5600-t0 for detailed flash informations

Specifications:
SOC: MT7986b
RAM: 512MB
Flash: 512 MB SPI NAND
Ports: 2 LAN 2.5Gbps (GPY211C)
WIFI: MT7976GN + MT7976AN
LED: 3 bicolor LED - 1 monocolor LED
Buttons: Reset and WPS

We can install all with U-boot and mtk_uartboot.
Load Uboot:

```
./mtk_uartboot -a -p ./mt7986-ram-ddr3-bl2.bin -s /dev/ttyUSB0 -f openwrt-mediatek-filogic-zyxel_wx5600-t0-ubootmod-bl31-uboot.fip
```
**WARNING: Please use a GBIT ethernet or force it on system**
**WARNING: Please use only LAN2 port in Uboot**
Press 0 on Bootmenu
```
mtd erase ubi
run ubi_format
bootmenu
```
Load and write BL2 and U-boot:
```
8
7
```

Load and write recovery and production
```
6
5
```

Signed-off-by: Valerio 'ftp21' Mancini <ftp21@ftp21.eu>
Co-authored-by: Hal Martin <halmartin@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18364
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>