ath79: add support for Sitecom WLR-7100

ath79: add support for Sitecom WLR-7100

Sitecom WLR-7100 v1 002 (marketed as X7 AC1200) is a dual band wireless
router.

Specification
SoC: Atheros AR1022
RAM: 64 MB DDR2
Flash: 8 MB SPI NOR
WIFI: 2.4 GHz 2T2R integrated
      5 GHz 2T2R QCA9882 integrated (connected to PCIe lane)
Ethernet: 5x 10/100/1000 Mbps QCA8337N
USB: 1x 2.0
LEDS: 4x GPIO controlled, 5x switch
Buttons: 2x GPIO controlled
UART: row of 4 unpopulated holes near USB port, starting count from
      white triangle on PCB
      1. VCC 3.3V, 2. GND, 3. TX, 4. RX
      baud: 115200, parity: none, flow control: none

Installation
1. Connect to one of LAN (yellow) ethernet ports,
2. Open router configuration interface,
3. Go to Toolbox > Firmware,
4. Browse for OpenWrt factory image with dlf extension and hit Apply,
5. Wait few minutes, after the Power LED will stop blinking, the router
   is ready for configuration.

Known issues
5GHz LED doesn't work

Additional information
When TX line on UART is connected, and board is switched on from power
off state, the DDR memory training may fail.
If connected to UART, when prompted for number on boot, one can enter
number 4 to open bootloader (U-Boot) command line.
OEM firmware shell password is: SitecomSenao
useful for creating backup of original firmware.
There is also another revision of this device (v1 001), which may or may
not work with introduced images.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>