test: spi: Add sandbox_spi_get_{speed, mode} interface

Introduce sandbox_spi_get_{speed, mode} public interface to retrieve the
sandbox spi bus internal state. They are meant to be used in sandbox spi
testcases.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>