Seaboard has a couple of external USB ports. Add the platform devices and
GPIO infra-structure to enable these.

I tested both ports with a mouse on Seaboard non-clamshell.

This change is extracted from code in the ChromeOS 2.6.38 kernel.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>