gpio: sandbox: Track whether a GPIO is driven

Add a new flag to keep track of whether sandbox is driving the pin, or
whether it is expecting an input signal. If it is driving, then the value
of the pin is the value being driven (0 or 1). If not driving, then we
consider the value 0, since we don't currently handle things like pull-ups
yet.

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