ath79: add support for 8devices Carambola2 development board

ath79: add support for 8devices Carambola2 development board

Specifications:
- Atheros AR9331 (400 MHz)
- 64 MB of RAM (DDR2)
- 16 MB of Flash (SPI)
- 1T1R 2.4 Wlan (AR9331)
- 2x 10/100 Mbps Ethernet
- 3x LEDs, 1x gpio button
- 1x USB 2.0, 5V
- UART over usb, 115200n8

Upgrading from ar71xx target:
- Put image into board:
    scp openwrt-ath79-generic-8dev_carambola2-squashfs-sysupgrade.bin \
        root@192.168.1.1/tmp/
- Run sysupgrade
    sysupgrade /tmp/sysupgrade.bin

Upgrading from u-boot:
- Set up tftp server with sysupgrade.bin image
- Go to u-boot (reboot and press ESC when prompted)
- Set TFTP server IP
    setenv serverip 192.168.1.254
- Set device ip from same subnet
    setenv ipaddr 192.168.1.1
- Copy new firmware to board
    tftpboot 0x81000000 sysupgrade.bin
- erase flash
    erase 0x9f050000 +${filesize}
- flash firmware
    cp.b 0x81000000 0x9f050000 ${filesize}
- Reset board
    reset

Signed-off-by: Rytis Zigmantavičius <rytis.z@8devices.com>
[wrapped long line in commit description, whitespace and art address
 fix in DTS, keep default lan/wan setup, removed -n in sysupgrade]
Signed-off-by: Petr Štetiar <ynezz@true.cz>