ath79: add support for ALFA Network Pi-WiFi4

ath79: add support for ALFA Network Pi-WiFi4

ALFA Network Pi-WiFi4 is a Qualcomm QCA9531 v2 based, high-power 802.11n
WiFi board in Raspberry Pi 3B shape, equipped with 1x FE and 4x USB 2.0.

Specifications:

- Qualcomm/Atheros QCA9531 v2
- 650/400/200 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16+ MB of flash (SPI NOR)
- 1x 10/100 Mbps Ethernet
- 2T2R 2.4 GHz Wi-Fi with Qorvo RFFM8228P FEM
- 2x IPEX/U.FL connectors on PCB
- 4x USB 2.0 Type-A
- Genesys Logic GL850G 4-port USB HUB
- USB power is controlled by GPIO
- 5x LED (3x on PCB, 2x in RJ45, 4x driven by GPIO)
- 1x button (reset)
- external h/w watchdog (EM6324QYSP5B, enabled by default)
- 1x micro USB Type-B for power and system console (Holtek HT42B534)
- UART and GPIO (8-pin, 1.27 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>