ath79: add support for ALFA Network Tube-2HQ

ALFA Network Tube-2HQ is a successor of the Tube-2H/P series (EOL) which
was based on the Atheros AR9331. The new version uses Qualcomm QCA9531.

Specifications:

- Qualcomm/Atheros QCA9531 v2
- 650/400/200 MHz (CPU/DDR/AHB)
- 64 or 128 MB of RAM (DDR2)
- 16+ MB of flash (SPI NOR)
- 1x 10/100 Mbps Ethernet with passive PoE input (24 V)
  (802.3at/af PoE support with optional module)
- 1T1R 2.4 GHz Wi-Fi with external PA (SE2623L, up to 27 dBm) and LNA
- 1x Type-N (male) antenna connector
- 6x LED (5x driven by GPIO)
- 1x button (reset)
- external h/w watchdog (EM6324QYSP5B, enabled by default)
- UART (4-pin, 2.00 mm pitch) header on PCB

Flash instruction:

You can use sysupgrade image directly in vendor firmware which is based
on LEDE/OpenWrt. Alternatively, you can use web recovery mode in U-Boot:

1. Configure PC with static IP 192.168.1.2/24.
2. Connect PC with one of RJ45 ports, press the reset button, power up
   device, wait for first blink of all LEDs (indicates network setup),
   then keep button for 3 following blinks and release it.
3. Open 192.168.1.1 address in your browser and upload sysupgrade image.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>