mediatek: add support for Airtel AirFiber AAP4221ZY

mediatek: add support for Airtel AirFiber AAP4221ZY

The Airtel AirFiber AAP4221ZY is an ISP-provided router that is part
of the ISP's Fixed Wireless Access (FWA) solution. The FWA system
consists of two units: an outdoor 5G unit and this indoor Wi-Fi router
connected via a PoE ethernet link. This commit adds support for the
indoor unit.

The OEM appears to be Zyxel with model number EX3310-T0, although this
is not printed on any label and only found in the vendor firmware DTS,
bootlog, and other internal references.

Hardware
--------
SOC: MediaTek MT7981B
CPU: 2 core Cortex-A53 (1300MHz)
RAM: 512MB DDR3
FLASH: 512MB SPI-NAND (Micron, 4096 page, 256KiB block)
WIFI: MediaTek MT7976 802.11ax 2x2 2.4GHz + 5GHz
ETH: Airoha AN8855 GbE Switch (2x LAN) + MT7981 internal GbE PHY (WAN)
POE: WAN port provides PoE to the outdoor unit (GPIO 10)
UART: 3V3 115200 8N1

Bootloader
----------
The device uses BL2 -> U-Boot -> Zyxel zloader chain. U-Boot has a
hardcoded command to always launch zloader (similar to the Zyxel
EX5601-T0, see commit 1c05388ab04c). zloader is the Zyxel secondary
bootloader which handles dual-boot (ubi/ubi2 A/B scheme), firmware
verification, and PoE port enablement.

zloader requires a UBI volume named "zyfwinfo" containing a 256-byte
metadata structure with a "ZYXE" magic header and a valid checksum.
Without this volume, zloader refuses to boot the firmware. The
zyfwinfo structure also contains a sequence number (range 0-9999,
wraps around) used to select which firmware bank to boot - the bank
with the higher sequence number wins. This OpenWrt image sets it to
5555 to always win over stock firmware starting at 0.

During sysupgrade, platform.sh refreshes the zyfwinfo volume before
writing kernel and rootfs to UBI, as rootfs_data is sized to fill the
remaining space.

Note that OpenWrt can only be flashed to ubi (mtd6). The zloader
appends rootubi=ubi or rootubi=ubi2 to the kernel bootargs based on
which partition it selects, but OpenWrt cannot understand this
parameter, so the firmware must always reside on the first bank (ubi).

UART access
-----------
Serial console output is restricted by the U-Boot environment variable
EngDebugFlag which is 0x00 by default. Users need to first obtain root
access on the vendor firmware and run:

  fw_setenv EngDebugFlag 0x01

Then reboot. Connect UART and press Enter to enter the zloader shell
(ZHAL> prompt). Use the ATGU command to switch to the U-Boot shell.
Note: ATGU must be entered twice to get the MT7981> prompt:

  ZHAL> ATGU
  [zloader reloads]
  ZHAL> ATGU
  MT7981>

Installation
------------
1. Enable UART access as described above.
2. Enter U-Boot shell (ATGU twice from ZHAL> prompt).
3. Load the initramfs image via TFTP:

  MT7981> setenv ipaddr 192.168.1.1
  MT7981> setenv serverip 192.168.1.2
  MT7981> tftpboot 0x46000000 openwrt-mediatek-filogic-airtel_aap4221zy-initramfs-kernel.bin
  MT7981> bootm 0x46000000

4. Once OpenWrt boots from initramfs, transfer and flash the
   sysupgrade image:

  sysupgrade -n /tmp/openwrt-mediatek-filogic-airtel_aap4221zy-squashfs-sysupgrade.bin

5. The device will reboot and zloader will boot OpenWrt from flash.

Signed-off-by: Ahmed Naseef <naseefkm@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22809
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>