ath79: add support for ALFA Network N2Q

ath79: add support for ALFA Network N2Q

ALFA Network N2Q is an outdoor N300 AP/CPE based on Qualcomm/Atheros
QCA9531 v2. This model is a successor of the old N2 which was based
on Atheros AR7240. FCC ID: 2AB8795311.

Specifications:

- Qualcomm/Atheros QCA9531 v2
- 650/400/200 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of flash (SPI NOR)
- 2T2R 2.4 GHz Wi-Fi with ext. PA (Skyworks SE2623L) and LNA
- 2x 10/100 Mbps Ethernet with passive PoE input in one port (24 V)
- PoE pass through in second port (controlled by GPIO)
- support for optional 802.3af/at PoE module
- 1x mini PCIe slot (PCIe bus, extra 4.2 V for high power cards)
- 2x IPEX/U.FL connectors on PCB
- 1x USB 2.0 mini Type-B (power controlled by GPIO)
- 8x LED (7 of them are driven by GPIO)
- 1x button (reset)
- external h/w watchdog (EM6324QYSP5B, enabled by default)
- UART (4-pin, 2.54 mm pitch) header on PCB
- LEDs (2x 5-pin, 2.54 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>