ath79: support for TP-Link EAP225-Wall v2

ath79: support for TP-Link EAP225-Wall v2

TP-Link EAP225-Wall v2 is an AC1200 (802.11ac Wave-2) wall plate access
point. UART access and debricking require fine soldering.

The device was kindly provided for porting by Stijn Segers.

Device specifications:
* SoC: QCA9561 @ 775MHz
* RAM: 128MiB DDR2
* Flash: 16MiB SPI-NOR (GD25Q127CSIG)
* Wireless 2.4GHz (SoC): b/g/n, 2x2
* Wireless 5Ghz (QCA9886): a/n/ac, 2x2 MU-MIMO
* Ethernet (SoC): 4× 100Mbps
  * Eth0 (back): 802.3af/at PoE in
  * Eth1, Eth2 (bottom)
  * Eth3 (bottom): PoE out (can be toggled by GPIO)
* One status LED
* Two buttons (both work as failsafe)
  * LED button, implemented as KEY_BRIGHTNESS_TOGGLE
  * Reset button

Flashing instructions, requires recent firmware (tested on 1.20.0):
* ssh into target device and run `cliclientd stopcs`
* Upgrade with factory image via web interface

Debricking:
* Serial port can be soldered on PCB J4 (1: TXD, 2: RXD, 3: GND, 4: VCC)
    * Bridge unpopulated resistors R162 (TXD) and R165 (RXD)
      Do NOT bridge R164
    * Use 3.3V, 115200 baud, 8n1
* Interrupt bootloader by holding CTRL+B during boot
* tftp initramfs to flash via sysupgrade or LuCI web interface

MAC addresses:
MAC address (as on device label) is stored in device info partition at
an offset of 8 bytes. ath9k device has same address as ethernet, ath10k
uses address incremented by 1.
From OEM ifconfig:
    br0       Link encap:Ethernet  HWaddr 50:...:04
    eth0      Link encap:Ethernet  HWaddr 50:...:04
    wifi0     Link encap:UNSPEC  HWaddr 50-...-04-...
    wifi1     Link encap:UNSPEC  HWaddr 50-...-05-...

Signed-off-by: Sander Vanheule <sander@svanheule.net>
[fix IMAGE_SIZE]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>