gpio: sandbox: Fully separate pin value from output value

At present we have the concept of a pin's external value. This is what
is used when getting the value of a pin. But we still set the
GPIOD_IS_OUT_ACTIVE flag when changing the value. This is not actually
correct, since if the pin changes from output to input, the external
value need not change. Adjust the logic for this difference.

Signed-off-by: Simon Glass <sjg@chromium.org>

Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>