Added default CS behaviour for SSI slaves. SSI devices can set a property
to enable CS behaviour which will create a GPIO on the device which is the
CS. Tristating of the bus on SSI transfers is implemented.

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>