ath79: add support for TP-Link TL-WA801ND v3/v4

ath79: add support for TP-Link TL-WA801ND v3/v4

This ports support for these devices from ar71xx.

Specification:

- System-On-Chip: Qualcomm Atheros QCA9533
- CPU/Speed: v3: 560 MHz, v4: 650 MHz
- Flash: 4096 KiB
- RAM: 32 MiB
- Ethernet: 1 port @ 100M
- Wireless: SoC-integrated: QCA9533 2.4GHz 802.11bgn

In contrast to the implementation in ar71xx (reset and WiFi button),
the device actually features reset and WPS buttons.

Flashing instructions:

Upload the ...-factory.bin file via OEM web interface.

TFTP Recovery:

1. Set PC to fixed IP address 192.168.0.66
2. Download *-factory.bin image and rename it to
   wa801ndv3_tp_recovery.bin
3. Start a tftp server with the image file in its root directory
4. Turn off the router
5. Press and hold Reset button
6. Turn on router with the reset button pressed and wait ~15 seconds
7. Release the reset button and after a short time
   the firmware should be transferred from the tftp server
8. Wait ~30 second to complete recovery.

TFTP recovery has only been tested with v3, and the Wiki states
that the procedure won't work for v4, which cannot be verified
or falsified at the moment.

Tested by Tim Ward (see forum):
https://forum.openwrt.org/t/ath79-support-for-tp-link-tl-wa901nd-v3-v4-v5/61246/13

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 9a477b833ab2aea96b9eee55acb5f9e7b01b36d8)