gpio: sandbox: Add GPIOD_IS_AF for gpio configured in alternate function

This allows to test if a pin's label if displayed using gpio_get_status()
when this pin is configured in alternate function.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>