ath79: add suport for EnGenius EPG5000

ath79: add suport for EnGenius EPG5000

EnGenius EPG5000 (v1.0.0, marketed as IoT Gateway) is a dual band
wireless router.

Specification
SoC:		Qualcomm Atheros QCA9558
RAM:		256 MB DDR2
Flash:		16 MB SPI NOR
WIFI:		2.4 GHz 3T3R integrated
		5 GHz 3T3R QCA9880 Mini PCIe card
Ethernet:	5x 10/100/1000 Mbps QCA8337N
USB:		1x 2.0
LEDS:		4x GPIO controlled
Buttons:	2x GPIO controlled
UART:		4 pin header, 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 Tools > Firmware,
4. Select 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.

Alternative installation
1. Prepare TFTP server with OpenWrt sysupgrade image,
2. Connect to one of LAN (yellow) ethernet ports,
3. Connect to UART port (leaving out VCC pin!),
4. Power on router,
5. When asked to enter a number 1 or 3 hit 2, this will select flashing
   image from TFTP server option,
6. You'll be prompted to enter TFTP server ip (default is 192.168.99.8),
   then router ip (default is 192.168.99.9) and for last, image name
   downloaded from TFTP server (default is uImageESR1200_1750),
7. After providing all information U-Boot will start flashing the image,
   You can observe progress on console, it'll take few minutes and when
   the Power LED will stop blinking, router is ready for configuration.

Additional information
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: aigo3d0a0tdagr
useful for creating backup of original firmware.
When doing upgrade from OpenWrt ar71xx image, it is recomended to not keep
the old configuration.

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