ath79: add support for TP-Link EAP245-v3

ath79: add support for TP-Link EAP245-v3

TP-Link EAP245 v3 is an AC1750 (802.11ac Wave-2) ceiling mount access
point. UART access (for debricking) requires non-trivial soldering.

Specifications:
* SoC: QCA9563 (CPU/DDR/AHB @ 775/650/258 MHz)
* RAM: 128MiB
* Flash: 16MiB SPI-NOR
* Wireless 2.4GHz (SoC): b/g/n 3x3
* Wireless 5GHz (QCA9982): a/n/ac 3x3 with MU-MIMO
* Ethernet (QCA8337N switch): 2× 1GbE, ETH1 (802.3at PoE) and ETH2
* Green and amber status LEDs
* Reset switch (GPIO, available for failsafe)

Flashing instructions:
All recent firmware versions (latest is 2.20.0), can disable firmware
signature verification and use a padded firmware file to flash OpenWrt:
* ssh into target device and run `cliclientd stopcs`
* upload factory image via web interface

The stopcs-method is supported from firmware version 2.3.0. Earlier
versions need to be upgraded to a newer stock version before flashing
OpenWrt.

Factory images for these devices are RSA signed by TP-Link. While the
signature verification can be disabled, the factory image still needs to
have a (fake) 1024 bit signature added to pass file checks.

Debricking instructions:
You can recover using u-boot via the serial port:
* Serial port is available from J3 (1:TX, 2:RX, 3:GND, 4:3.3V)
* Bridge R237 to connect RX, located next to J3
* Bridge R225 to connect TX, located inside can on back-side of board
* Serial port is 115200 baud, 8n1, interrupt u-boot by holding ctrl+B
* Upload initramfs with tftp and upgrade via OpenWrt

Device mac addresses:
Stock firmware has the same mac address for 2.4GHz wireless and
ethernet, 5GHz is incremented by one. The base mac address is stored in
the 'default-mac' partition (offset 0x90000) at an offset of 8 bytes.
ART blobs contain no mac addresses.
From OEM ifconfig:
    ath0      Link encap:Ethernet  HWaddr 74:..:E2
    ath10     Link encap:Ethernet  HWaddr 74:..:E3
    br0       Link encap:Ethernet  HWaddr 74:..:E2
    eth0      Link encap:Ethernet  HWaddr 74:..:E2

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>