ar71xx: add support for ALFA Network R36A

ar71xx: add support for ALFA Network R36A

ALFA Network R36A is a successor of the previous model, the R36 (Ralink
RT3050F based). New version is based on Qualcomm/Atheros QCA9531 v2.

Specification:

- 650/400/200 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 2x 10/100 Mbps Ethernet, with passive PoE support (24 V)
- 2T2R (QCA9531) 2.4 GHz, 2x u.fl connectors on PCB
- 1x USB 2.0 (power controlled by GPIO)
- 6x LED (5 of them are driven by GPIO)
- 2x button (reset, wifi/wps)
- external h/w watchdog (EM6324QYSP5B, disabled and not used)
- DC jack for main power input (12 V)
- UART 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>