mediatek: filogic: add support for Bazis AX3000WM

mediatek: filogic: add support for Bazis AX3000WM

 - SoC: MediaTek MT7981B
 - CPU: 2x 1.3 GHz Cortex-A53
 - Flash: 128 MiB SPI NAND (Winbond W25N01GV)
 - RAM: 256 MiB (Nanya NT5CC128M16JR-EK)
 - WLAN: 2.4 GHz, 5 GHz (MediaTek MT7976CN, 802.11ax)
 - Ethernet: 1x 10/100/1000/2500 Mbps MaxLinear GPY211C WAN, 1x10/100/1000 Mbps MT7981 LAN
 - USB 3.0 port
 - 2x buttons: Reset, Mesh
 - 3x LEDs on top panel (red, green, blue)
 - 3x PHY port LEDs (1 amber, 2 green)
 - Board version: WD830M V2.0
 - Power: 5 VDC, 3 A

UART: internal test points, 3V3 115200 8N1 (RX, TX, GND)

Interface	MAC			Algorithm
LAN (label)	1c:bf:ce:xx:xx:x1	0x4
WAN		1c:bf:ce:xx:xx:x2	0xa
WLAN 2.4G	1c:bf:ce:xx:xx:x3	0x2a
WLAN 5G		1c:bf:ce:xx:xx:x4	0x24

At the moment installation is possible via UART only since SSH root
is password protected.

Set a static ip on the ethernet interface of your PC:
(ip address: 192.168.1.2, subnet mask:255.255.255.0).
Boot into initramfs via TFTP:
```
setenv serverip 192.168.1.2
tftpboot 0x46000000 openwrt-mediatek-filogic-bazis_ax3000wm-initramfs-recovery.itb
bootm 0x46000000
```

Install kmod-mtd-rw and activate it:
```
apk update && apk add kmod-mtd-rw
insmod mtd-rw i_want_a_brick=1
```
Upload to the router and write OpenWrt BL2 and FIP bootloader:
```
mtd write openwrt-mediatek-filogic-bazis_ax3000wm-preloader.bin BL2
mtd write openwrt-mediatek-filogic-bazis_ax3000wm-bl31-uboot.fip FIP
```
Prepare ubi partition:
```
ubidetach -p /dev/mtd4
ubiformat /dev/mtd4 -y
ubiattach -p /dev/mtd4
```

Upgrade via luci web interface with sysupgrade file or by issuing a command:
```
sysupgrade -n openwrt-mediatek-filogic-bazis_ax3000wm-squashfs-sysupgrade.itb
```
Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21813
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>