mediatek: add support for ZyXEL NWA50AX Pro

mediatek: add support for ZyXEL NWA50AX Pro

Hardware
--------
CPU:    Mediatek MT7981
RAM:    512M DDR4
FLASH:  256M NAND
ETH:    MaxLinear GPY211 (2.5GbE N Base-T)
WiFi:   Mediatek MT7981 (2.4GHz 2T2R:2 5GHz 3T3R:2 802.11ax)
BTN:    1x Reset
LED:    1x Multi-Color

UART Console
------------
Available below the rubber cover next to the ethernet port.

Settings: 115200 8N1

Layout:

<12V> <LAN> GND-RX-TX-VCC

Logic-Level is 3V3. Don't connect VCC to your UART adapter!

Installation Web-UI
-------------------
Upload the Factory image using the devices Web-Interface.

As the device uses a dual-image partition layout, OpenWrt can only
installed on Slot A. This requires the current active image prior
flashing the device to be on Slot B.

In case this is not the case, OpenWrt will boot only one time, returning
to the ZyXEL firmware the second boot.

If this happens, first install a ZyXEL firmware upgrade of any version
and install OpenWrt after that.

Installation TFTP / Recovery
----------------------------
This installation routine is especially useful in case of a bricked
device.

Attach to the UART console header of the device. Interrupt the boot
procedure by pressing Enter.

The bootloader has a reduced command-set available from CLI, but more
commands can be executed by abusing the atns command.

Boot a OpenWrt initramfs image available on a TFTP server at
192.168.1.66. Rename the image to nwa50axpro-openwrt-initramfs.bin.

 $ atnf nwa50axpro-openwrt-initramfs.bin
 $ atna 192.168.1.88
 $ atns "192.168.1.66; tftpboot; setenv fdt_high 0xffffffffffffffff;
   bootm"

Upon booting, set the booted image to the correct slot:

 $ zyxel-bootconfig /dev/mtd9 get-status
 $ zyxel-bootconfig /dev/mtd9 set-image-status 0 valid
 $ zyxel-bootconfig /dev/mtd9 set-active-image 0

Copy the OpenWrt sysupgrade image to the device using scp.
Write the sysupgrade image to NAND using sysupgrade.

 $ sysupgrade -n image.bin

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit f0445746f6fd96fc7c5394b238153bd2ff22bc5b)