ramips: mt7621: add TP-Link EAP235-Wall support

The TP-Link EAP235-Wall is a wall-mounted, PoE-powered AC1200 access
point with four gigabit ethernet ports.

When connecting to the device's serial port, it is strongly advised to
use an isolated UART adapter. This prevents linking different power
domains created by the PoE power supply, which may damage your devices.

The device's U-Boot supports saving modified environments with
`saveenv`. However, there is no u-boot-env partition, and saving
modifications will cause the partition table to be overwritten. This is
not an issue for running OpenWrt, but will prevent the vendor FW from
functioning properly.

Device specifications:
* SoC: MT7621DAT
* RAM: 128MiB
* Flash: 16MiB SPI-NOR
* Wireless 2.4GHz (MT7603EN): b/g/n, 2x2
* Wireless 5GHz (MT7613BEN): a/n/ac, 2x2
* Ethernet: 4× GbE
  * Back side: ETH0, PoE PD port
  * Bottom side: ETH1, ETH2, ETH3
* Single white device LED
* LED button, reset button (available for failsafe)
* PoE pass-through on port ETH3 (enabled with GPIO)

Datasheet of the flash chip specifies a maximum frequency of 33MHz, but
that didn't work. 20MHz gives no errors with reading (flash dump) or
writing (sysupgrade).

Device mac addresses:
Stock firmware uses the same MAC address for ethernet (on device label)
and 2.4GHz wireless. The 5GHz wireless address is incremented by one.
This address is stored in the 'info' ('default-mac') partition at an
offset of 8 bytes.
From OEM ifconfig:
    eth     a4:2b:b0:...:88
    ra0     a4:2b:b0:...:88
    rai0    a4:2b:b0:...:89

Flashing instructions:
* Enable SSH in the web interface, and SSH into the target device
* run `cliclientd stopcs`, this should return "success"
* upload the factory image via the web interface

Debricking:
U-boot can be interrupted during boot, serial console is 57600 baud, 8n1
This allows installing a sysupgrade image, or fixing the device in
another way.
* Access serial header from the side of the board, close to ETH3,
  pin-out is (1:TX, 2:RX, 3:GND, 4:3.3V), with pin 1 closest to ETH3.
* Interrupt bootloader by holding '4' during boot, which drops the
  bootloader into its shell
* Change default 'serverip' and 'ipaddr' variables (optional)
* Download initramfs with `tftpboot`, and boot image with `bootm`
    # tftpboot 84000000 openwrt-initramfs.bin
    # bootm

Revert to stock:
Using the tplink-safeloader utility from the firmware-utils package,
TP-Link's firmware image can be converted to an OpenWrt-compatible
sysupgrade image:
  $ ./staging_dir/host/bin/tplink-safeloader -B EAP235-WALL-V1 \
      -z EAP235-WALLv1_XXX_up_signed.bin -o eap235-sysupgrade.bin

This can then be flashed using the OpenWrt sysupgrade interface. The
image will appear to be incompatible and must be force flashed, without
keeping the current configuration.

Known issues:
- DFS support is incomplete (known issue with MT7613)
- MT7613 radio may stop responding when idling, reboot required.
  This was an issue with the ddc75ff704 version of mt76, but appears to
  have improved/disappeared with bc3963764d.
  Error notice example:
  [ 7099.554067] mt7615e 0000:02:00.0: Message 73 (seq 1) timeout

Hardware was kindly provided for porting by Stijn Segers.

Tested-by: Stijn Segers <foss@volatilesystems.org>
Signed-off-by: Sander Vanheule <sander@svanheule.net>

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