ssi: Move ssi.h into a separate directory

Move the ssi.h include file into the ssi directory.

While touching the code also fix the typdef lines as
checkpatch complains.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
