ar71xx: add support for 8devices Rambutan development board

ar71xx: add support for 8devices Rambutan development board

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

This commit adds basic support for Rambutan development kit

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
- 1x USB2.0 host socket + 1x USB2.0 pins on 2.54mm header

Flash instructions:
Stock firmware is OpenWrt, so use:
 sysupgrade -n /tmp/lede-ar71xx-nand-rambutan-squashfs-sysupgrade.tar
or upgarde from GUI (don't save config)
Use factory image to flash from U-Boot:
 tftpboot 80060000 lede-ar71xx-nand-rambutan-squashfs-factory.ubi
 nand erase.part ubi
 nand write 80060000 ubi ${filesize}

Signed-off-by: Mantas Pucka <mantas@8devices.com>
[split support in uboot-envtools package into a separate commit,
fixed alphabetical order in lib/preinit/05_set_iface_mac_ar71xx]
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>