ath79: add support for TP-Link EAP225 v1

ath79: add support for TP-Link EAP225 v1

TP-Link EAP225 v1 is an AC1200 (802.11ac Wave-1) ceiling mount access point.

Device specifications:
* SoC: QCA9563 @ 775MHz
* RAM: 128MiB DDR2
* Flash: 16MiB SPI-NOR
* Wireless 2.4GHz (SoC): b/g/n, 2x2
* Wireless 5Ghz (QCA9882): a/n/ac, 2x2
* Ethernet (AR8033): 1× 1GbE, 802.3at PoE

Flashing instructions:
* Ensure the device is upgraded to firmware v1.4.0
* Exploit the user management page in the web interface to start telnetd
  by changing the username to `;/usr/sbin/telnetd -l/bin/sh&`.
* Immediately change the malformed username back to something valid
  (e.g. 'admin') to make ssh work again.
* Use the root shell via telnet to make /tmp world writeable (chmod 777)
* Extract /usr/bin/uclited from the device via ssh and apply the binary
  patch listed below. The patch is required to prevent `uclited -u` in
  the last step from crashing.
* Copy the patched uclited binary back to the device at /tmp/uclited
  (via ssh)
* Upload the factory image to /tmp/upgrade.bin (via ssh)
* Run `chmod +x /tmp/uclited && /tmp/uclited -u` to install OpenWrt.

uclited patching:
    --- xxd uclited
    +++ xxd uclited-patched
    @@ -53811,7 +53811,7 @@
     000d2330: 8c44 0000 0320 f809 0000 0000 8fbc 0010  .D... ..........
     000d2340: 8fa6 0a4c 02c0 2821 8f82 87c4 0000 0000  ...L..(!........
    -000d2350: 8c44 0000 0c13 461c 27a7 0018 8fbc 0010  .D....F.'.......
    +000d2350: 8c44 0000 2402 0000 0000 0000 8fbc 0010  .D..$...........
     000d2360: 1040 001d 0000 1821 8f99 8378 3c04 0058  .@.....!...x<..X
     000d2370: 3c05 0056 2484 ad68 24a5 9f00 0320 f809  <..V$..h$.... ..

To make sure the correct file is patched, the following MD5 checksums
should match the unpatched and patched files:
    4bd74183c23859c897ed77e8566b84de  uclited
    4107104024a2e0aeaf6395ed30adccae  uclited-patched

Debricking:
* Serial port can be soldered on unpopulated 4-pin header
  (1: TXD, 2: RXD, 3: GND, 4: VCC)
    * Bridge unpopulated resistors running from pins 1 (TXD) and 2 (RXD).
      Do NOT bridge the pull-down for pin 2, running parallel to the
      header.
    * Use 3.3V, 115200 baud, 8n1
* Interrupt bootloader by holding CTRL+B during boot
* tftp initramfs to flash via the LuCI web interface
    setenv ipaddr 192.168.1.1 # default, change as required
    setenv serverip 192.168.1.10 # default, change as required
    tftp 0x80800000 initramfs.bin
    bootelf $fileaddr

Tested by forum user KernelMaker.

Link: https://forum.openwrt.org/t/eap225-v1-firmware/87116
Signed-off-by: Sander Vanheule <sander@svanheule.net>