ath79: add support for 8devices Rambutan board

ath79: add support for 8devices Rambutan board

Rambutan is a Wifi module based on QCA9550/9557
http://www.8devices.com/products/rambutan

Specification:

- 720/600/200 MHz (CPU/DDR/AHB)
- 128 MB of DDR2 RAM
- 128 MB of NAND Flash
- 1x 100Mbps Ethernet
- 1x 1000Mbps Ethernet (PHY on dev-kit)
- 1x Wifi radio 2x2 MIMO, dualband 2.4 and 5 GHz
- 2x U.FL connectors on module, chip antennas on dev-kit
- 1x miniPCIe slot
- 2x USB2.0 host
- 2x UART
- SPI, I2C, GPIO

Flash instructions:

Upgrade from ar71xx target:
 sysupgrade -F /tmp/openwrt-ath79-nand-8dev_rambutan-squashfs-sysupgrade.tar
or upgrade from GUI (don't save config)

Use factory image to flash from U-Boot:
 tftpboot 80060000 openwrt-ath79-nand-8dev_rambutan-squashfs-factory.ubi
 nand erase.part ubi
 nand write 80060000 ubi ${filesize}

Signed-off-by: Petr Štetiar <ynezz@true.cz> [copy&pasted missing commit description]
Signed-off-by: Andrey Bondar <a.bondar@8devices.com>