ar71xx: add support for TP-Link TL-WR902AC v1

ar71xx: add support for TP-Link TL-WR902AC v1

TP-Link TL-WR902AC v1 is a pocket-size, dual-band (AC750), successor of
TL-MR3020 (both devices use very similar enclosure, in same size). New
device is based on Qualcomm QCA9531 v2 + QCA9887. FCC ID: TE7WR902AC.

Specification:

- 650/391/216 MHz (CPU/DDR/AHB)
- 1x 10/100 Mbps Ethernet
- 1x USB 2.0 (GPIO-controlled power)
- 64 MB of RAM (DDR2)
- 8 MB of FLASH
- 2T2R 2.4 GHz (QCA9531)
- 1T1R 5 GHz (QCA9887)
- 5x LED (GPIO-controlled), 2x button, 1x 3-pos switch
- UART pads on PCB (TP1 -> TX, TP2 -> RX, TP3 -> GND, TP4 -> 3V3, jumper
  resitors are missing on TX/RX lines)
- 1x micro USB (for power only)

Flash instructions:

Use "factory" image under vendor GUI.

Recovery instructions:

This device contains tftp recovery mode inside U-Boot. You can use it to
flash LEDE (use "factory" image) or vendor firmware.

1. Configure PC with static IP 192.168.0.66/24 and tftp server.
2. Rename "lede-ar71xx-generic-tl-wr902ac-v1-squashfs-factory.bin"
   to "wr902acv1_un_tp_recovery.bin" and place it in tftp server dir.
3. Connect PC with LAN port, press the reset button, power up the router
   and keep button pressed until WPS LED lights up.
4. Router will download file from server, write it to flash and reboot.

Root access over serial line in vendor firmware: root/sohoadmin.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>