mediatek: add support for Wavlink WL-WNT100X3 ubootmod

mediatek: add support for Wavlink WL-WNT100X3 ubootmod

This allows us to use the full size of nand, which increases ubi size
from 90M to 122.25M.

Flashing instructions:
1. Login into the device and backup all your partitions,
especially `Factory` and 'HW' to be able to come back to stock and use all
Wavlink services.

2. Unlock mtd partitions:
```
apk update && apk add kmod-mtd-rw
insmod mtd-rw i_want_a_brick=1
```
3. Upload new `bl2` and `fip` to the router /tmp and write them:
```
mtd write /tmp/openwrt-mediatek-filogic-wavlink_wl-wnt100x3-ubootmod-preloader.bin bl2
mtd write /tmp/openwrt-mediatek-filogic-wavlink_wl-wnt100x3-ubootmod-bl31-uboot.fip fip
mtd erase ubi
```
4. Set static IP on your PC:
   IP 192.168.1.254/24, GW 192.168.1.1
5. Serve OpenWrt initramfs image using TFTP server.
6. Cut off the power and re-engage, wait for TFTP recovery to complete.
7. After OpenWrt initramfs has booted, prepare ubi and envs:
```
ubidetach -p /dev/mtd4 && ubiformat /dev/mtd4 -y && ubiattach -p /dev/mtd4 && ubimkvol /dev/ubi0 -n 0 -N ubootenv -s 128KiB && ubimkvol /dev/ubi0 -n 1 -N ubootenv2 -s 128KiB
```
8. Perform sysupgrade.

Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22753
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>