mediatek: add support for netis NX32U

mediatek: add support for netis NX32U

This PR adds support for netis NX32U router.

Specification
-------------
- SoC       : MediaTek MT7981BA dual-core ARM Cortex-A53 1.3 GHz
- RAM       : 256 MiB DDR3
- Flash     : SPI-NAND 128 MiB (ESMT)
- WLAN      : MediaTek MT7976CN dual-band WiFi 6
  - 2.4 GHz : b/g/n/ax, MIMO 2x2
  - 5 GHz   : a/n/ac/ax, MIMO 2x2
- Ethernet  : 10/100/1000 Mbps x3 (LAN, MediaTek MT7531AE)
              10/100/1000 Mbps x1 (WAN, SoC internal phy)
- USB       : 3.0
- Buttons   : Mesh, Reset
- LEDs      : 1x Power (green), unmanaged
              1x Internet (green), gpio-controlled
              1x WPS (green), gpio-controlled
              1x WiFi 2.4 GHz (green), gpio-controlled
              1x WiFi 5 GHz (green), gpio-controlled
              1x LAN activity (green), switch-controlled
              1x WAN activity (green), switch-controlled
              1x USB (green), gpio-controlled
- Power     : 12 VDC, 1 A

Installation
------------
1. Connect to the router using ssh (user: admin, pass: web interface
   password)
2. Backup:
```
cat /dev/mtd0 | gzip -1 -c > /tmp/mtd0_spi0.0.bin.gz
cat /dev/mtd1 | gzip -1 -c > /tmp/mtd1_BL2.bin.gz
cat /dev/mtd2 | gzip -1 -c > /tmp/mtd2_u-boot-env.bin.gz
cat /dev/mtd3 | gzip -1 -c > /tmp/mtd3_Factory.bin.gz
cat /dev/mtd4 | gzip -1 -c > /tmp/mtd4_FIP.bin.gz
cat /dev/mtd5 | gzip -1 -c > /tmp/mtd5_ubi.bin.gz
```
3. Download mtd backup from the /tmp dir of the router to your PC using
   scp protocol
4. Upload OpenWrt 'bl31-uboot.fip', 'preloader.bin' images to the /tmp
   dir of the router using scp protocol
5. Write FIP and BL2 (replace bootloader):
```
mtd write /tmp/openwrt-mediatek-filogic-netis_nx32u-bl31-uboot.fip FIP
mtd write /tmp/openwrt-mediatek-filogic-netis_nx32u-preloader.bin BL2
```
6. Place OpenWrt
   'openwrt-mediatek-filogic-netis_nx32u-initramfs-recovery.itb' image on
   the tftp server (IP: 192.168.1.254)
7. Erase 'ubi' partition and reboot the router:
   mtd erase ubi
   reboot
8. U-Boot automatically boot OpenWrt recovery image from tftp server to
   the RAM
9. Upload OpenWrt 'sysupgrade.itb' image to the /tmp dir of the router
   (IP: 192.168.1.1) using scp protocol
10. Connect to the router using ssh and run:
```
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
sysupgrade -n openwrt-mediatek-filogic-netis_nx32u-squashfs-sysupgrade.itb
```

Recovery
--------
1. Place OpenWrt
   'openwrt-mediatek-filogic-netis_nx32u-initramfs-recovery.itb' image on
   the tftp server (IP: 192.168.1.254)
2. Press “Reset” button and power on the router. After ~10 sec release
   the button.
3. Use OpenWrt initramfs system for recovery

+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| LAN     | dc:xx:xx:d1:xx:18 | label     |
| WAN     | dc:xx:xx:d1:xx:1a | label+2   |
| WLAN 2g | de:xx:xx:11:xx:19 |           |
| WLAN 5g | de:xx:xx:71:xx:19 |           |
+---------+-------------------+-----------+
The LAN MAC was found in 'Factory', 0x1fef20
The WAN MAC was found in 'Factory', 0x1fef26
The WLAN 2g/5g MAC prototype was found in 'Factory', 0x4

Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21368
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>