ath79: support for TP-Link EAP225 v4

ath79: support for TP-Link EAP225 v4

This model is almost identical to the EAP225 v3.
Major difference is the RTL8211FS PHY Chipset.

Device specifications:
* SoC: QCA9563 @ 775MHz
* RAM: 128MiB DDR2
* Flash: 16MiB SPI-NOR
* Wireless 2.4GHz (SoC): b/g/n, 3x3
* Wireless 5Ghz (QCA9886): a/n/ac, 2x2 MU-MIMO
* Ethernet (RTL8211FS): 1× 1GbE, 802.3at PoE

Flashing instructions:
* 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 R225 (TXD) and R237 (RXD).
      Do NOT bridge R230.
    * Use 3.3V, 115200 baud, 8n1
* Interrupt bootloader by holding CTRL+B during boot
* tftp initramfs to flash via 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

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.

Signed-off-by: Sven Hauer <sven.hauer+github@uniku.de>