ssi: Name the CS GPIO

To get it out of the default GPIO list. This allows child devices to
use the un-named GPIO namespace without having to be SSI aware. That
is, there is no more need for machines to know about the obscure
policy where GPIO 0 is the SSI chip-select and GPIO 1..N are the
concrete class GPIOs (defined locally as 0..N-1).

This is most notable in stellaris, which uses a device which has both
SSI and concrete level GPIOs.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>