ath79: add support for 8Devices Carambola3 board

ath79: add support for 8Devices Carambola3 board

Carambola3 is a WiFi module based on Qualcomm/Atheros QCA4531
http://wiki.8devices.com/carambola3

Specification:

    - 650/600/216 MHz (CPU/DDR/AHB)
    - 128 MB of RAM (DDR2)
    - 32 MB of FLASH
    - 2T2R 2.4 GHz
    - 2x 10/100 Mbps Ethernet
    - 1x USB 2.0 Host socket
    - UART for serial console
    - 12x GPIO

Flash instructions:

    Upgrading from ar71xx target:
    - Upload image into the board:
        scp openwrt-ath79-generic-8dev_carambola3-squashfs-sysupgrade.bin \
          root@192.168.1.1/tmp/
    - Run sysupgrade
        sysupgrade -F /tmp/openwrt-ath79-generic-8dev_carambola3-squashfs-sysupgrade.bin

Upgrading from u-boot:
    - Set up tftp server with openwrt-ath79-generic-8dev_carambola3-initramfs-kernel.bin
    - Go to u-boot (reboot and press ESC when prompted)
    - Set TFTP server IP
        setenv serverip 192.168.1.254
    - Set device ip from the same subnet
        setenv ipaddr 192.168.1.1
    - Copy new firmware to board
        tftpboot 0x82000000 initramfs.bin
    - Boot OpenWRT
        bootm 0x82000000
    - Upload image openwrt-ath79-generic-8dev_carambola3-squashfs-sysupgrade.bin into the board
    - Run sysupgrade.

Signed-off-by: Andrey Bondar <a.bondar@8devices.com>
Link: https://github.com/openwrt/openwrt/pull/15514
Signed-off-by: Robert Marko <robimarko@gmail.com>