ath79: add support for jjPlus JA76PF2

jjPlus JA76PF2 (marketed as IntellusPro2) is a network embedded board.

Specification
SoC:		Atheros AR7161
RAM:		64 MB DDR
Flash:		16 MB SPI NOR
Ethernet:	2x 10/100/1000 Mbps AR8316
		LAN (CN11), WAN/PoE (CN6 - close to power barrel
		connector, 48 V)
MiniPCI:	2x
LEDS:		4x, which 3 are GPIO controlled
Buttons:	2x GPIO controlled
		Reset (SW1, closer to ethernet ports), WPS (SW2)
Serial:		1x (only RX and TX are wired)
		baud: 115200, parity: none, flow control: none

Currently there is one caveat compared to ar71xx target images as the
MAC addresses are random on every reboot. To remedy this one needs to
store the WAN MAC address in RedBoot configuration. OpenWrt on first
boot, after flashing, will read out the address and assign proper ones
to both WAN and LAN ports. It is iportant to NOT keep the old
configuration when doing sysupgrade from ar71xx.

Upgrading from OpenWrt ar71xx image
1. Connect to serial port,
2. Download OpenWrt sysupgrade image to /tmp directory and flash it
   with:
    sysupgrade -n <openwrt_sysupgrade_image_name>
3. After writing new image OpenWrt will reboot, now interrupt boot
   process and enter RedBoot (bootloader) command line by pressing
   Ctrl+C,
4. Enter following commands (replace variable accordingly),
    set_mac (to view MAC addresses)
    alias ethaddr <wan_port_mac_adress>
    (confirm storing the value by inputting y and pressing Enter)
    reset
5. Now board should restart and boot OpenWrt with proper MAC addresses.

Installation
1. Prepare TFTP server with OpenWrt initramfs image,
2. Connect to WAN ethernet port,
3. Connect to serial port,
4. Power on the board and enter RedBoot (bootloader) command line by
   pressing Ctrl+C,
5. Enter following commands (replace variables accordingly):
    set_mac (to view MAC addresses)
    alias ethaddr <wan_port_mac_address>
    (confirm storing the value by inputting y and pressing Enter)
    ip_adress -l <board_ip_adress>/24 -h <tftp_server_ip_adress>
    load -r -b 0x80060000 <openwrt_initramfs_image_name>
    exec -c ""
6. Now board should boot OpenWrt initramfs image,
7. Download OpenWrt sysupgrade image to /tmp directory and flash it
   with:
    sysupgrade <openwrt_sysupgrade_image_name>
8. Wait few minutes, after the D2 LED will stop blinking, the board
   is ready for configuration.

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