ath79: add support for ALFA Network R36A

ath79: 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, FCC
ID: 2AB879531.

Support for this device was first introduced in af8f0629df (ar71xx
target). When updating from previous release (and/or ar71xx target),
user should only adjust the WAN LED trigger type (netdev in ar71xx,
switch port in ath79).

Specifications:

- Qualcomm/Atheros QCA9531 v2
- 650/400/200 MHz (CPU/DDR/AHB)
- 128 MB (R36AH/-U2) or 64 MB (R36A) of RAM (DDR2)
- 16 MB of flash (SPI NOR)
- 2x 10/100 Mbps Ethernet
- Passive PoE input support (12~36 V) in RJ45 near DC jack
- 2T2R 2.4 GHz Wi-Fi with Qorvo RFFM8228P FEM
- 2x IPEX/U.FL connectors on PCB
- 1x USB 2.0 Type-A
- 1x USB 2.0 mini Type-B in R36AH-U2 version
- USB power is controlled by GPIO
- 6/7x LED (5/6 of them are driven by GPIO)
- 2x button (reset, wifi/wps)
- external h/w watchdog (EM6324QYSP5B, enabled by default)
- DC jack with lock, for main power input (12 V)
- UART (4-pin, 2.54 mm pitch) header on PCB

Optional/additional features in R36A series (R36A was the first model):
- for R36AH:    USB 2.0 hub*
- for R36AH-U2: USB 2.0 hub*, 1x USB 2.0 mini Type-B, one more LED

*) there are at least three different USB 2.0 hub in R36AH/-U2 variants:
- Terminus-Tech FE 1.1
- Genesys Logic GL852G
- Genesys Logic GL850G (used in latests revision)

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>