zynq: spi: Honour the activation / deactivation delay

This is not currently implemented. Add support for this so that the
Chrome OS EC can be used reliably.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>