ar71xx: add support for TP-Link TL-WR942N v1

ar71xx: add support for TP-Link TL-WR942N v1

TP-Link TL-WR942N v1 is a 2.4 GHz single-band N450 router, based on
Qualcomm/Atheros QCA9561.

Specification:

- 775/650/258 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 3T3R 2.4 GHz
- 5x 10/100 Mbps Ethernet
- 2x USB 2.0
- 11x LED (most are controlled by 74HC595)
- 2x button
- UART header on PCB*

* Serial console is disabled in OEM non-beta firmwares and corresponding
  GPIO pins 14 and 15 are assigned to control USB1 and USB2 LEDs by
  production (non-beta) U-Boot and firmware.

Currently not working:

1. USB1 and USB2 LEDs if UART RX and TX pins are assigned to their GPIOs
   by some U-Boot versions.

Flash instruction under vendor GUI:

1. Download "lede-ar71xx-generic-tl-wr942n-v1-squashfs-factory.bin".
2. Go to WEB interface and perform usual firmware upgrade.

FLash instruction under U-Boot recovery mode (doesn't work in beta
firmware):

1. Setup PC with static IP "192.168.0.66/24" and tftp server.
2. Change "*-factory" image filename to "WR942v1_recovery.bin" and make
   it available to download from your tftp server.
3. Press "reset" button and power up the router, wait till "WPS" LED
   turns on.

Flash instruction under U-Boot, using UART (can be done only with
preinstalled UART-enabled U-Boot version!):

1. Use "tpl" to stop autobooting and obtain U-Boot CLI access.
2. Setup ip addresses for U-Boot and your tftp server.
3. Issue below commands:
   tftp 0x81000000 lede-ar71xx-generic-tl-wr942n-v1-sysupgrade.bin
   erase 0x9f020000 +$filesize
   cp.b 0x81000000 0x9f020000 $filesize
   reset

Signed-off-by: Serg Studzinskii <serguzhg@gmail.com>
[minor code style fixes, extended commit message]
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>