ARM: kirkwood: Add Plat'Home OpenBlocks A6 support

Add support for Plat'Home OpenBlocks A6 using the device tree
where possible.
This commit supports SATA, USB, ether and serial console.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>