ARM: plat-orion: add missing ehci include to common.h

Removing orion ehci include from board files will raise a compiler
error because plat/common.h is using an enum provided by orion ehci
but not including the include itself. This just adds the missing include.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>